1 + 2 + 3 + 4 + ... + 46 + 47 + 48 + 49 + 50 = (1 + 50) + (2 + 49) + ... + (25 + 26) = 51 + 51 + ... + 51 = 51 * 25 = 1275

Вычисления:
$\begin{array}{r}\times \underset{¯}{\begin{array}{r}51\\ 25\end{array}}\\ 255\\ 102\phantom{0}\\ \overline{1275}\end{array}$

5 + 10 + 15 + ... + 85 + 90 + 95 + 100 = (5 + 100) + (10 + 95) + (15 + 90) + ... + (50 + 55) = 105 + 105 + 105 + ... + 105 = 105 * 10 = 1050