how to differentiate a function and then evaluate at a predefined variable?

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sam plant 2018-11-6

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https://ww2.mathworks.cn/matlabcentral/answers/428312-how-to-differentiate-a-function-and-then-evaluate-at-a-predefined-variable

评论： madhan ravi 2018-11-6

If i wanted to differentiate f=x*y with respect to y and then evaluate at a value y = 6 say. how would i do this? i would write the code syms y f=x*y g=diff(f) but then how would i go about the evaluation because it seems y is now fixed as a symbol. Also, i have this as predefined input values x,y so i would like to use the y value predefined as i'm writing it in a function

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madhan ravi 2018-11-6

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https://ww2.mathworks.cn/matlabcentral/answers/428312-how-to-differentiate-a-function-and-then-evaluate-at-a-predefined-variable#answer_345487

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    syms x y
    f=x*y
    A=diff(f,y)
    subs(A,y,6)

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sam plant 2018-11-6

编辑：sam plant 2018-11-6

Thank you! I used that example to just keep it simple, my actual equation was involving logs and 3 variables but i just wanted t keep the principle the same. It's working after reading this, i appreciate the help!