The problem is as follows: Differentiate y = cos(a^3 + x^3)
my current solution:
By the chain rule
u = a^3 + x^3
f(u) = cos(u)
u' = 3a^2 + 3x^2 (by the power rule)
f ' (u) = -sin(u)
therefore,
-sin(a^3 + x^3)(3a^2 + 3x^2) = -3a^2sin(a^3 + x^3) - 3x^2sin(a^3 + x^3)
This is the solution I come up with, however it is incorrect and I don't know why.
correct solution should be -3x^2sin(a^3 + x^3). Where is the -3a^2sin(a^3 + x^3)
going?