# Mean Value Theorem homework

1. Aug 28, 2006

If $$f$$ is continuous and $$\int^{3}_{1} f(x) dx = 8$$, show that $$f$$ takes on the value 4 at least once on the interval [1,3] . I know that the average value of $$f(x)$$ is 4. So does this imply that $$f_{ave} = f(c) = 4$$ and $$f(x)$$ takes on the value of 4 at least once on the interval [1,3] ?

2. Aug 28, 2006

### quasar987

I'd go with contradiction. Suppose there is no x0 in [1,3] such that f(x0)=4. With that assumption, what does the mean value thm says about the values f can take on [1,3]? There are two cases, etc.

3. Aug 28, 2006

### 0rthodontist

Yes, but not directly--the mean value theorem is about derivatives. What is the derivative of g(x) = $$\int_{3}^{x} f(t) dt$$?

4. Aug 28, 2006

### quasar987

make that the intermediate value thm, sorry.

Anyway, courtigrad's way is a little bit faster, more elegant and "on topic" provided this is an exercice designed to make you apply the mean valut thm.

Last edited: Aug 28, 2006
5. Aug 28, 2006

Thanks for your help. Lets say I want to prove (not rigorously) the Mean Value Theorem for Integrals, $$\int_{a}^{b} f(x)\dx = f(c)(b-a)$$ by using the Mean Value Theorem for Derivatives to the function $$F(x) = \int^{x}_{a} f(t)\dt$$. We know that $$F(x)$$ is continous on [a,x] and differentiable on (a,x) . So $$F(x) - F(a) = f(c)(x-a)$$ or $$\frac{\int^{b}_{a} f(t)\dt - \int_{a}^{a} f(t)\dt}{b-a} = f(c)(b-a)$$. So $$f(c) = \frac{1}{b-a}\int_{a}^{b} f(t)\dt$$. Is this correct?

Thanks

6. Aug 28, 2006

### quasar987

minus the extra (b-a) in the second to last equation, I'd say this is a perfectly acceptable and rigourous proof of the mean value thm for integral. Nice job!

7. Aug 28, 2006

### 0rthodontist

This step
$$\frac{\int^{b}_{a} f(t)\dt - \int_{a}^{a} f(t)\dt}{b-a} = f(c)(b-a)$$
is not true. Also instead of just writing equations, you should explain what you're doing--for example you should say, "where c is some value between a and b" and you should say when you use the mean value theorem or fundamental theorem of calculus to justify a step.

Because of the way you've presented it, I'm not sure if you are already aware of this, but what is d(F(x))/dx? What is F(a) and F(b)?

8. Aug 28, 2006

$$\frac{dF(x)}{dx} = f(t)$$. $$F(a) = 0$$ and $$F(b) = \int_{a}^{b} f(t)$$

9. Aug 28, 2006

### quasar987

(you rock!)

But I agree with ortho on the last part: in an exam, you'd most certainly lose points for not expliciting your reasoning in terms of the condition of applicability of the thm, etc. For instance, the step that goes,

should read "We know by one version of the fond thm of calculus that under the assumption of the continuity of f on [a,b], $$F(x)$$ is continous on [a,b] , differentiable on (a,b) and that F'(x)=f(x). Therefor, by the mean value thm, there exists a number c in (a,b) such that F(b)-F(a)=F'(c)(b-a). But F'(c) = f(c), $F(b)=\int_a^b f(t)dt$, F(a)=$\int_a^a f(t)dt=0$, hence the result."

Last edited: Aug 28, 2006
10. Aug 29, 2006

### HallsofIvy

Staff Emeritus
Let $F(x)= \int_1^x f(t)dt$. Then $F(1)= \int_1^1 f(t)dt= 0$ and $F(3)= \int_1^3 f(t)dt= 8$ so $\frac{F(3)- F(1)}{3- 1}= \frac{8- 0}{2}= 4$. By the mean value theorem then, there must exist c between 1 and 3 such that F'(c)= f(c)= 4.