# Composite function troubles

#### mitchconnor

##### New member
g(x)=−4x2−5x
f(x)=−3x2+7x−5(g(x))

f(g(−1))=?

First, let's solve for the value of the inner function, g(−1). Then we'll know what to plug into the outer function.

g(−1)=−4(−1)2+(−5)(−1)

I don't understand why they transformed the minus symbol into an addition symbol. This has happened a few times now. Every time I think I get an answer, I get hoodwinked by this change!

Help would be much appreciated.

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#### MarkFL

##### Administrator
Staff member
Re: Composite function troubles~

Note that:

$$\displaystyle -4(-1)^2+(-5)(-1)=-4(-1)^2-5(-1)$$

I would choose to write it they way it is on the right, but both are equivalent. It boils down to the fact that subtracting a number is the same as adding the negative of that number:

$$\displaystyle a-b=a+(-b)$$

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