# Homework Help: Use the generalised mean value theorem to prove this

1. Apr 27, 2013

### chipotleaway

1. The problem statement, all variables and given/known data
Let f(x) be a continuous function on [a, b] and differentiable on (a, b). Using the generalised mean value theorem, prove that:

$$f(x)=f(c) + (x-c)f'(c)+\frac{(x-c)^2}{2}f''(\theta)$$ for some $$\theta \in (c, x)$$

2. Relevant equations
Hints given suggest consdiering F(x) = f(x) - f(c) - f'(c)(x-c) and G(x) = (x-a)^2
(F and G and their derivatives are the functions that appear in the given mean value theorem)

3. The attempt at a solution
I don't have no clue as to how to proceed other than making the substitutions in the hints and plugging it into the MVT equation and hoping the function I'm after pops out but this doesn't seem right - especially for the number of marks it's worth (plus I'm not really getting anywhere with it anyway).

2. Apr 27, 2013

### micromass

What is the generalized mean value theorem?

3. Apr 27, 2013

### chipotleaway

Cauchy's mean value theorem. If F and G are continuous on [a, b] and differentiable on (a, b) and G'(x)≠0 for all x in (a, b) then there exists some c in (a, b) such that:

$$\frac{F'(c)}{G'(c)}=\frac{F(b)-F(a)}{G(b)-G(a)}$$

4. Apr 27, 2013

### micromass

OK, then you should just plug in your functions $F(x)=f(x)-f(x)-f^\prime(c)(x-c)$ and $G(x) = (x-c)^2$ into this mean-value theorem.

Note however, that the numbers $c$, $a$ and $b$ in this mean-value theorem are not the same as the ones in the OP!

5. Apr 27, 2013

### chipotleaway

Thanks! I'll have to check my algebra then, haha.

6. Apr 28, 2013

### chipotleaway

If $F(x)=f(x)-f(c)-f'(c)(x-c)$, then when I differentiate, do I treat f(c) and f'(c) as constants? I'm getting $F'(x)=f'(x)-f''(c)(x-c)+f'(c)$

7. Apr 28, 2013

### micromass

Yes. They are constants.