How to substitute x by an integer in a derivative function

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

Answer 1

[Andreas Apostolatos]

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.

More information about function 'subs' you can find in the following documentation page,

https://www.mathworks.com/help/symbolic/subs.html

I hope that this helps.

Kind Regards,

Andreas

[Rodrigo Toledo]

thanks mate. a(2) is a typo. i meant a(1), sorry.

[Paul]

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)