Why can't I differentiate $ x^{sin x} $ using the power rule?












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I'm trying to differentiate $ x^{sin x} $, with respect to $ x $ and $x > 0 $. My textbook initiates with $ y = x^{sin x} $, takes logarithms on both sides and arrives at the answer $$ x^{sin x - 1}.sin x + x^{sin x}.cos x log x $$



Why can't I use the power rule to proceed like this: $ y' = (sin x) x^{sin x-1} cos x $.



Here, I've first differentiated $ x $ with respect to $ sin x $ and then I've differentiated $ sin x $ with respect to $ x $ to get $ cos x $.










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    4












    $begingroup$


    I'm trying to differentiate $ x^{sin x} $, with respect to $ x $ and $x > 0 $. My textbook initiates with $ y = x^{sin x} $, takes logarithms on both sides and arrives at the answer $$ x^{sin x - 1}.sin x + x^{sin x}.cos x log x $$



    Why can't I use the power rule to proceed like this: $ y' = (sin x) x^{sin x-1} cos x $.



    Here, I've first differentiated $ x $ with respect to $ sin x $ and then I've differentiated $ sin x $ with respect to $ x $ to get $ cos x $.










    share|cite|improve this question









    $endgroup$















      4












      4








      4





      $begingroup$


      I'm trying to differentiate $ x^{sin x} $, with respect to $ x $ and $x > 0 $. My textbook initiates with $ y = x^{sin x} $, takes logarithms on both sides and arrives at the answer $$ x^{sin x - 1}.sin x + x^{sin x}.cos x log x $$



      Why can't I use the power rule to proceed like this: $ y' = (sin x) x^{sin x-1} cos x $.



      Here, I've first differentiated $ x $ with respect to $ sin x $ and then I've differentiated $ sin x $ with respect to $ x $ to get $ cos x $.










      share|cite|improve this question









      $endgroup$




      I'm trying to differentiate $ x^{sin x} $, with respect to $ x $ and $x > 0 $. My textbook initiates with $ y = x^{sin x} $, takes logarithms on both sides and arrives at the answer $$ x^{sin x - 1}.sin x + x^{sin x}.cos x log x $$



      Why can't I use the power rule to proceed like this: $ y' = (sin x) x^{sin x-1} cos x $.



      Here, I've first differentiated $ x $ with respect to $ sin x $ and then I've differentiated $ sin x $ with respect to $ x $ to get $ cos x $.







      derivatives






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      asked Dec 25 '18 at 11:29









      WorldGovWorldGov

      324111




      324111






















          4 Answers
          4






          active

          oldest

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          3












          $begingroup$

          The power rule states that the derivative of a function of the form $x^k$ is $kx^{k-1}$ where $k$ is a real number. In your case, $sin x$ is variable, and can takes values from $-1$ to $+1$, so is not constant.





          To prove the power rule, note that $y=x^kimplies ln y=kln ximplies frac{y'}y=frac kxcdot$ via the chain rule. Therefore, $y'=x^kfrac kx=kx^{k-1}$. When $K=sin x$, we need to do some more to manipulate the RHS, since $(sin xln x)'ne frac{sin x}x$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Does that proof hold for $k lt 0$?
            $endgroup$
            – DavidG
            Jan 6 at 11:44












          • $begingroup$
            Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
            $endgroup$
            – DavidG
            Jan 6 at 11:48










          • $begingroup$
            No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
            $endgroup$
            – DavidG
            Jan 6 at 11:50










          • $begingroup$
            I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
            $endgroup$
            – DavidG
            Jan 6 at 11:53












          • $begingroup$
            @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
            $endgroup$
            – TheSimpliFire
            Jan 6 at 11:54





















          1












          $begingroup$

          $(x^k)'=kx^{k-1},$ where $k$ is constant.



          In our case $sin{x}neq constant$ and we can not use this rule.



          By the way,
          $$left(x^{sin{x}}right)'=left(e^{sin{x}ln{x}}right)'=e^{sin{x}ln{x}}(sin{x}ln{x})'=...$$
          Can you end it now?






          share|cite|improve this answer











          $endgroup$





















            1












            $begingroup$

            Because the power rule requires that the exponent be a constant. $sin x$ is most definitely not a constant.






            share|cite|improve this answer









            $endgroup$





















              0












              $begingroup$

              You are trying to use the rule
              $$frac{d}{dx}f(g(x)) = g'(x)f'(g(x))$$



              But this rule doesn't apply here, because $x^{sin x}$ is not just a function of $sin x$, it is a function of $x$ and $sin x$.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                $endgroup$
                – Michael Rozenberg
                Dec 25 '18 at 11:49











              Your Answer





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              4 Answers
              4






              active

              oldest

              votes








              4 Answers
              4






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

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              3












              $begingroup$

              The power rule states that the derivative of a function of the form $x^k$ is $kx^{k-1}$ where $k$ is a real number. In your case, $sin x$ is variable, and can takes values from $-1$ to $+1$, so is not constant.





              To prove the power rule, note that $y=x^kimplies ln y=kln ximplies frac{y'}y=frac kxcdot$ via the chain rule. Therefore, $y'=x^kfrac kx=kx^{k-1}$. When $K=sin x$, we need to do some more to manipulate the RHS, since $(sin xln x)'ne frac{sin x}x$.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Does that proof hold for $k lt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:44












              • $begingroup$
                Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
                $endgroup$
                – DavidG
                Jan 6 at 11:48










              • $begingroup$
                No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:50










              • $begingroup$
                I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:53












              • $begingroup$
                @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
                $endgroup$
                – TheSimpliFire
                Jan 6 at 11:54


















              3












              $begingroup$

              The power rule states that the derivative of a function of the form $x^k$ is $kx^{k-1}$ where $k$ is a real number. In your case, $sin x$ is variable, and can takes values from $-1$ to $+1$, so is not constant.





              To prove the power rule, note that $y=x^kimplies ln y=kln ximplies frac{y'}y=frac kxcdot$ via the chain rule. Therefore, $y'=x^kfrac kx=kx^{k-1}$. When $K=sin x$, we need to do some more to manipulate the RHS, since $(sin xln x)'ne frac{sin x}x$.






              share|cite|improve this answer









              $endgroup$













              • $begingroup$
                Does that proof hold for $k lt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:44












              • $begingroup$
                Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
                $endgroup$
                – DavidG
                Jan 6 at 11:48










              • $begingroup$
                No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:50










              • $begingroup$
                I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:53












              • $begingroup$
                @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
                $endgroup$
                – TheSimpliFire
                Jan 6 at 11:54
















              3












              3








              3





              $begingroup$

              The power rule states that the derivative of a function of the form $x^k$ is $kx^{k-1}$ where $k$ is a real number. In your case, $sin x$ is variable, and can takes values from $-1$ to $+1$, so is not constant.





              To prove the power rule, note that $y=x^kimplies ln y=kln ximplies frac{y'}y=frac kxcdot$ via the chain rule. Therefore, $y'=x^kfrac kx=kx^{k-1}$. When $K=sin x$, we need to do some more to manipulate the RHS, since $(sin xln x)'ne frac{sin x}x$.






              share|cite|improve this answer









              $endgroup$



              The power rule states that the derivative of a function of the form $x^k$ is $kx^{k-1}$ where $k$ is a real number. In your case, $sin x$ is variable, and can takes values from $-1$ to $+1$, so is not constant.





              To prove the power rule, note that $y=x^kimplies ln y=kln ximplies frac{y'}y=frac kxcdot$ via the chain rule. Therefore, $y'=x^kfrac kx=kx^{k-1}$. When $K=sin x$, we need to do some more to manipulate the RHS, since $(sin xln x)'ne frac{sin x}x$.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Dec 25 '18 at 11:30









              TheSimpliFireTheSimpliFire

              12.7k62461




              12.7k62461












              • $begingroup$
                Does that proof hold for $k lt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:44












              • $begingroup$
                Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
                $endgroup$
                – DavidG
                Jan 6 at 11:48










              • $begingroup$
                No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:50










              • $begingroup$
                I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:53












              • $begingroup$
                @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
                $endgroup$
                – TheSimpliFire
                Jan 6 at 11:54




















              • $begingroup$
                Does that proof hold for $k lt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:44












              • $begingroup$
                Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
                $endgroup$
                – DavidG
                Jan 6 at 11:48










              • $begingroup$
                No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:50










              • $begingroup$
                I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
                $endgroup$
                – DavidG
                Jan 6 at 11:53












              • $begingroup$
                @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
                $endgroup$
                – TheSimpliFire
                Jan 6 at 11:54


















              $begingroup$
              Does that proof hold for $k lt 0$?
              $endgroup$
              – DavidG
              Jan 6 at 11:44






              $begingroup$
              Does that proof hold for $k lt 0$?
              $endgroup$
              – DavidG
              Jan 6 at 11:44














              $begingroup$
              Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
              $endgroup$
              – DavidG
              Jan 6 at 11:48




              $begingroup$
              Sorry, my apologies, I meant to type what if $x^k lt 0$ - can you still take the logarithm? If not, does this then only hold for $x gt 0$ ?
              $endgroup$
              – DavidG
              Jan 6 at 11:48












              $begingroup$
              No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
              $endgroup$
              – DavidG
              Jan 6 at 11:50




              $begingroup$
              No, I appreciate that - but just for clarity - this method as presented only holds for $x gt 0$?
              $endgroup$
              – DavidG
              Jan 6 at 11:50












              $begingroup$
              I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
              $endgroup$
              – DavidG
              Jan 6 at 11:53






              $begingroup$
              I agree, I'm not disputing that. Nor any I challenging that the derivative as you find holds for $x lt 0$. I'm just curious, in what you have presented in the solution - does it hold for all $x$? or is the reasoning (again as presented) only applicable for $x gt 0$?
              $endgroup$
              – DavidG
              Jan 6 at 11:53














              $begingroup$
              @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
              $endgroup$
              – TheSimpliFire
              Jan 6 at 11:54






              $begingroup$
              @DavidG I said 'Yes' to your comment already (so it only holds for $x>0$) :P
              $endgroup$
              – TheSimpliFire
              Jan 6 at 11:54













              1












              $begingroup$

              $(x^k)'=kx^{k-1},$ where $k$ is constant.



              In our case $sin{x}neq constant$ and we can not use this rule.



              By the way,
              $$left(x^{sin{x}}right)'=left(e^{sin{x}ln{x}}right)'=e^{sin{x}ln{x}}(sin{x}ln{x})'=...$$
              Can you end it now?






              share|cite|improve this answer











              $endgroup$


















                1












                $begingroup$

                $(x^k)'=kx^{k-1},$ where $k$ is constant.



                In our case $sin{x}neq constant$ and we can not use this rule.



                By the way,
                $$left(x^{sin{x}}right)'=left(e^{sin{x}ln{x}}right)'=e^{sin{x}ln{x}}(sin{x}ln{x})'=...$$
                Can you end it now?






                share|cite|improve this answer











                $endgroup$
















                  1












                  1








                  1





                  $begingroup$

                  $(x^k)'=kx^{k-1},$ where $k$ is constant.



                  In our case $sin{x}neq constant$ and we can not use this rule.



                  By the way,
                  $$left(x^{sin{x}}right)'=left(e^{sin{x}ln{x}}right)'=e^{sin{x}ln{x}}(sin{x}ln{x})'=...$$
                  Can you end it now?






                  share|cite|improve this answer











                  $endgroup$



                  $(x^k)'=kx^{k-1},$ where $k$ is constant.



                  In our case $sin{x}neq constant$ and we can not use this rule.



                  By the way,
                  $$left(x^{sin{x}}right)'=left(e^{sin{x}ln{x}}right)'=e^{sin{x}ln{x}}(sin{x}ln{x})'=...$$
                  Can you end it now?







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Dec 25 '18 at 11:45

























                  answered Dec 25 '18 at 11:38









                  Michael RozenbergMichael Rozenberg

                  107k1895199




                  107k1895199























                      1












                      $begingroup$

                      Because the power rule requires that the exponent be a constant. $sin x$ is most definitely not a constant.






                      share|cite|improve this answer









                      $endgroup$


















                        1












                        $begingroup$

                        Because the power rule requires that the exponent be a constant. $sin x$ is most definitely not a constant.






                        share|cite|improve this answer









                        $endgroup$
















                          1












                          1








                          1





                          $begingroup$

                          Because the power rule requires that the exponent be a constant. $sin x$ is most definitely not a constant.






                          share|cite|improve this answer









                          $endgroup$



                          Because the power rule requires that the exponent be a constant. $sin x$ is most definitely not a constant.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Dec 25 '18 at 11:55









                          The_SympathizerThe_Sympathizer

                          7,7402246




                          7,7402246























                              0












                              $begingroup$

                              You are trying to use the rule
                              $$frac{d}{dx}f(g(x)) = g'(x)f'(g(x))$$



                              But this rule doesn't apply here, because $x^{sin x}$ is not just a function of $sin x$, it is a function of $x$ and $sin x$.






                              share|cite|improve this answer









                              $endgroup$













                              • $begingroup$
                                Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                                $endgroup$
                                – Michael Rozenberg
                                Dec 25 '18 at 11:49
















                              0












                              $begingroup$

                              You are trying to use the rule
                              $$frac{d}{dx}f(g(x)) = g'(x)f'(g(x))$$



                              But this rule doesn't apply here, because $x^{sin x}$ is not just a function of $sin x$, it is a function of $x$ and $sin x$.






                              share|cite|improve this answer









                              $endgroup$













                              • $begingroup$
                                Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                                $endgroup$
                                – Michael Rozenberg
                                Dec 25 '18 at 11:49














                              0












                              0








                              0





                              $begingroup$

                              You are trying to use the rule
                              $$frac{d}{dx}f(g(x)) = g'(x)f'(g(x))$$



                              But this rule doesn't apply here, because $x^{sin x}$ is not just a function of $sin x$, it is a function of $x$ and $sin x$.






                              share|cite|improve this answer









                              $endgroup$



                              You are trying to use the rule
                              $$frac{d}{dx}f(g(x)) = g'(x)f'(g(x))$$



                              But this rule doesn't apply here, because $x^{sin x}$ is not just a function of $sin x$, it is a function of $x$ and $sin x$.







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 25 '18 at 11:39









                              TonyKTonyK

                              43k356135




                              43k356135












                              • $begingroup$
                                Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                                $endgroup$
                                – Michael Rozenberg
                                Dec 25 '18 at 11:49


















                              • $begingroup$
                                Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                                $endgroup$
                                – Michael Rozenberg
                                Dec 25 '18 at 11:49
















                              $begingroup$
                              Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                              $endgroup$
                              – Michael Rozenberg
                              Dec 25 '18 at 11:49




                              $begingroup$
                              Sometimes we can write $x=arcsinsin{x}$ and we'll obtain the function of $sin{x}.$ :)
                              $endgroup$
                              – Michael Rozenberg
                              Dec 25 '18 at 11:49


















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