#include <iostream>
#include <vector>
#include <numeric>
#include <cmath>
#include <cassert>
 
 
double cos(std::vector<double> const& lhs, std::vector<double> const& rhs) {
    assert(lhs.size() == rhs.size());
    return std::inner_product(lhs.begin(), lhs.end(), rhs.begin(), 0.)
            / (std::sqrt(std::inner_product(lhs.begin(), lhs.end(), lhs.begin(), 0.))
            * std::sqrt(std::inner_product(rhs.begin(), rhs.end(), rhs.begin(), 0.)));
}
 
 
int main() {
    std::cout << cos({1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, {10, 9, 8, 7, 6, 5, 4, 3, 2, 1}) << std::endl;
}
 

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