# How to substitute x by an integer in a derivative function

3 views (last 30 days)
Rodrigo Toledo on 29 Mar 2021
Commented: Paul on 30 Mar 2021
I have a function:
syms x
f(x) = (tan(x^2 + 3*x))^2
a = diff(f(x))
Now i want to calculate the a when x=1. How can i do it?
i tried a(2) but didnt work.
Thanks

Andreas Apostolatos on 29 Mar 2021
Hello,
To evaluate symbolic expression 'a' at 'x = 1' you can use function 'subs', namely,
a_at_1 = subs(a, 1);
I am not sure why you tried typing 'a(2)' since you are trying to evaluate 'a' at 1.
I hope that this helps.
Kind Regards,
Andreas
Rodrigo Toledo on 30 Mar 2021
thanks mate. a(2) is a typo. i meant a(1), sorry.
Paul on 30 Mar 2021
You can also define a to be a function of x and then evaluate it at the value of interest
>> syms x
f(x) = (tan(x^2 + 3*x))^2
a(x) = diff(f(x))
f(x) =
tan(x^2 + 3*x)^2
a(x) =
2*tan(x^2 + 3*x)*(2*x + 3)*(tan(x^2 + 3*x)^2 + 1)
>> a(1)
ans =
10*tan(4)*(tan(4)^2 + 1)

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