If $f(a)=f(b)=0$ and $|f''(x)|\le M$ prove $|\int_a^bf(x)\mathrm{d}x| \le \frac{M}{12}(b-a)^3$ If $f(a)=f(b)=0$ and $|f''(x)|\le M$. Prove 
$$|\int_a^bf(x)\mathrm{d}x| \le \frac{M}{12}(b-a)^3$$
I have thought about that since $f(a) = f(b) = 0 $ there is $\xi$ such that $f'(\xi) = 0$. Then when $x \le \xi$, $|f'(x)| \le (\xi - x)M$ and when $x \ge \xi$, $|f'(x)| \le (x-\xi)M$. After that $|f(x)| \le \frac{\xi^2 - (\xi -x)^2}{2}$ when $x \le \xi$ and $|f(x)| \le \frac{(b-\xi)^2 - (x - \xi)^2}{2}$ when $x \ge \xi$. Therefore 
$$|\int_a^b f(x)\mathrm{d}x| \le \int_a^\xi |f(x)| + \int_\xi^b |f(x)| = \frac{\xi^3}{3} + \frac{(b-\xi)^3}{3}$$
If $x = \frac{a+b}{2}$, we have
$$\frac{x^3}{3}+\frac{(b-x)^3}{3} = \frac{(b-a)^3}{12}$$
But in this situation $\frac{x^3}{3}+\frac{(b-x)^3}{3}$ is the minimal value. So I can't go on.

For simplicity let $a = -1, b = 1$. Let $\varepsilon > 0$. Consider the two functions $g_+(x) = \frac{M + \varepsilon}{2}(1-x^2)$ and $g_-(x) = -g_+(x)$. Then
$$
g_\pm(-1) = g_\pm(1) = 0, \; g_+''(x) = -M - \varepsilon, \; g_-''(x) = +M + \varepsilon
$$
and 
$$
\int_{-1}^1 g_+(x) = 2\frac{M+ \varepsilon}{3} = - \int_{-1}^1 g_-(x) dx \, .
$$
The claim now is that $g_-(x) \le f(x) \le g_+(x)$ for all $x$. 
Indeed, suppose $h_-(x) = f(x) - g_-(x)$ is negative somewhere in $(-1,1)$. Then  there is an absolute minimum $c \in (-1,1)$ at which $h_-''(c) \ge 0$ (second derivative test). But
$$
h_-''(c) = f''(x) - g_-''(c) = f''(c) - M - \varepsilon \le |f''(c)| - M - \varepsilon \le - \varepsilon < 0
$$
and therefore this is impossible. 
Consequently  $g_-(x) \le f(x)$ for all $x$. Similarly $f(x) \le g_+(x)$ for all $x$. It follows that
$$
-2\frac{M + \varepsilon}{3} \le \int_{-1}^1 f(x) dx \le 2\frac{M + \varepsilon}{3} \, .
$$
Since $\varepsilon > 0$ was arbitrary, the desired estimate follows. 
