#include <iostream>
#include <cmath>

double integrate(auto f, double a, double b, std::size_t n) {
  double dx = (b - a) / n;
  double acc = 0.;
  for (std::size_t i = 0; i < n; ++i) {
    double x = a + i * dx;
    acc += f(x) * dx;
  }
  return acc;
}

int main() {
  std::cout << integrate([](double x){ return x * std::sin(x); }, 0., M_PI, 10)
	    << std::endl;
}