>>770
double trapezium(double a, double b, int n) {
    int i;
    double x, w;
    w = b - a;
    x = (func1(a) + func1(b)) / 2;
    for(i = 1; i < n; i++) x += func1( a + w * i / n );
    return x * w / n;
}

simpsonと挿しかえれ