# Function

• Jan 10th 2009, 11:54 AM
nick_c
Function
I could really use some help with this function. It's probly really easy but I don't know what I'm doing wrong.

If f(x)=xsquared+3x, find f(2-a).

I did it like this.

f(x)=xsquared+3x
f(2-a)=(2-a)squared+3(2-a)
f(2-a)=4-asquared+6-3a
f(2-a)=-asquared-3a+10

But my teacher told us that it was.

-asquared-7a+10

If you could help me I would really appreciate it. Thanks!(Bow)
• Jan 10th 2009, 11:57 AM
Chris L T521
Quote:

Originally Posted by nick_c
I could really use some help with this function. It's probly really easy but I don't know what I'm doing wrong.

If f(x)=xsquared+3x, find f(2-a).

I did it like this.

f(x)=xsquared+3x
f(2-a)=(2-a)squared+3(2-a)
f(2-a)=4-asquared+6-3a
f(2-a)=-asquared-3a+10

But my teacher told us that it was.

-asquared-7a+10

If you could help me I would really appreciate it. Thanks!(Bow)

Note that $\displaystyle \left(2-a\right)^2\neq 2^2-a^2$!!!

It turns out that $\displaystyle \left(x-y\right)^2=x^2-2xy+y^2$. In your case, $\displaystyle x=2$ and $\displaystyle y=a$

Can you try to take it from here?