Posted by **Jeana** on Thursday, August 27, 2009 at 5:58pm.

f(4b)=4x^2+5x-5

f(x+1)=8x^2+x-3

f(-x)=8x^2+x-3

*I think that the first two use F.O.I.L but i still cant get it

- Math -
**MathMate**, Thursday, August 27, 2009 at 8:37pm
I'm afraid I do not catch your question.

Are you supposed to find f(4b) when f(x)=4x^2+5x-5? Or are you supposed to find f(x) when f(4b)=4x^2+5x-5. The second option does not make sense because the expression f(4b) should not leave any trace of "x" when x=4b.

Please clarify.

- Math -
**Jeana**, Thursday, August 27, 2009 at 10:16pm
x=4b sorry

- Math -
**MathMate**, Thursday, August 27, 2009 at 10:40pm
Would the question then be:

1.

If f(x)=4x^2+5x-5, find f(4b).

In this case,

f(4b)= 4(4b)² + 5(4b) -5

= 4(16b²) + 20b -5

= 64b² + 20b -5

- Math -
**Jeana**, Thursday, August 27, 2009 at 11:09pm
Okay thank you that really makes more since your a lifesaver! Do you know how to do number 2 and 3 that you could walk me through too? I have literally tried them 11 times and it still says they are wrong

- Math -
**MathMate**, Friday, August 28, 2009 at 12:29am
2.

First step: write down the function in terms of x:

f(x)=8x^2+x-3

Then substitute x by what is in the parentheses (x+1) in this case. Don't forget the parentheses everywhere you make a substitution:

f(x+1)=8(x+1)² + (x+1) - 3

Expand and collect terms as required:

f(x+1)

=8(x+1)² + (x+1) - 3

=8(x²+2x+1) + x+1 -3

=8x² + 16x + 8 +x+1 -3

=8x² + 17x + 6

Proceed the same way for number 3. Post for checking if necessary. The answer is 8x²-x-3.

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