# Math

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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 -

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.

• Math -

x=4b sorry

• Math -

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 -

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 -

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.