>>573
#include <math.h>
#include <stdio.h>
#define n 20
double f(double x) {
    return sin(2 * x) * cos(x);
}
int main(void) {
    int i;
    double a = 0., b = M_PI / 4, h = (b - a) / n, s = 0.;
    for (i = 1; i < n; ++i) s += f(a + i * h);
    s *= 2;
    s += f(0) + f(n);
    s *= h / 2;
    printf("%f", s);
    return 0;
}