# Math Help - find the dy/dx

1. ## find the dy/dx

if y=f(√(x²+9) and f'(5)=-2, find dy/dx when x=4

2. Originally Posted by hawtmonkey
if y=f(√(x²+9) and f'(5)=-2, find dy/dx when x=4
Look at it like $y=f(g[x])$, where g(x) is the square root. Now just use the normal chain rule to find $y'=f'(g[x])*g'[x]$. Notice that g(4)=5 so that's why that extra bit was given to you. So now all you have to do is find g'[5].