Изменения

|proof=

$\sigma(f, g, \tau) = \sum\limits_{i=0}^{n-1} f(\xi_k) (g(x_{k + 1}) - g(x_k)) = \\

\sum\limits_{j=1}^n f(\xi_{j-1}) g(x_j) - \sum\limits_{k=0}^{n-1} f(\xi_k) g(x_k) = \\

= f(\xi_{n-1}) g(x_n) - f(\xi_0) g(x_0) + \sum\limits_{j=1}^{n-1} g(x_j) (f(\xi_{j - 1}) - f(\xi_j)) = \\