>>132
もっと早く
double area_(double (*fn)(double), double x0, double x1, int n)
{
int i;
double s = 0.0, x = x0, dx = (x1-x0)/n;
for (i = 0; i < n; ++i) {
s += dx * (*fn)(x);
x += dx;
}
return s;
}