# Functional Equation

• Jan 18th 2010, 11:33 AM
Logic
Functional Equation
Greetings,

I am not quite sure what classification this problem should actually have, but my teacher, who was unable to help me with it, said it was a functional equation.

The aim is to find the values of a, b, c and d or represent the function on the first line like the one on the second line.
• Jan 18th 2010, 07:26 PM
TheEmptySet
Quote:

Originally Posted by Logic
Greetings,

I am not quite sure what classification this problem should actually have, but my teacher, who was unable to help me with it, said it was a functional equation.

The aim is to find the values of a, b, c and d or represent the function on the first line like the one on the second line.

This is just like finding the Taylor series of f center at x=1.

$f(x)=a(x-1)^3+b(x-1)^2+c(x-1)+d$

If we take a few derivatives we will see that

$f'(x)=3a(x-1)^2+2b(x-1)+c$

$f''(x)=6a(x-1)+2b$

$f'''(x)=6a$

Now compare this with the other functions derivatives

$f(x)=x^3+x^2-x+1$
$f'(x)=3x^2+2x-1$
$f''(x)=6x+2$
$f'''(x)=6$

Now compare the derivatives

$f'''(1)=6 =6a \implies a=1$

$f''(1)=6+2=2b \implies b=4$

$f'(1)=4=c \implies c=4$

$f(1)=2=d$

So we get

$f(x)=(x-1)^3+4(x-1)^2+4(x-1)+2$