# Derivatives of Composite Functions

#### Dough

I just need a nudge in the rigth direction ais dont know where to start
Let y = f(x^2 + 3x - 5) find dy/dx when x = 1, given that f'(-1) = 2

Thanks!

#### AKG

Homework Helper
Let g(x) = x² + 3x - 5, then y = f(g(x)). dy/dx = f'(g(x))g'(x)

#### Dough

i am not sure what else y have to do to get the answer, i wrote that out as well as

-1 = x^2 + 3x -5
solved and got x = 1 or -3...

but what else?

dy/dx = f'(x^2 + 3x - 5)(2x + 3)

#### HallsofIvy

Dough said:
i am not sure what else y have to do to get the answer, i wrote that out as well as
-1 = x^2 + 3x -5
solved and got x = 1 or -3...
but what else?
dy/dx = f'(x^2 + 3x - 5)(2x + 3)
I suggest going back and reading the problem again! You were asked to find y'(1). How about setting x= 1?

#### Jameson

Very good suggestion.

And just to ease your worries, you were given a good piece of information: that f'(-1)=2. Do you see where this applies to the problem?

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving