Question

Să se calculeze numerele
S=1+2+3+...+25
S=7+8+9+...+25
S=11+22+33+...+143​

Answer 1

Răspuns:

$s = \frac{(1 + 25) \times 25}{2} = \frac{26 \times 25}{2} = 13 \times 25 = 325 \\ s = \frac{(7 + 25) \times 19}{2} = \frac{32 \times 19}{2} = 16 \times 19 = 304 \\ s = \frac{(11 + 143) \times13 }{2} = \frac{154 \times 13}{2} = \frac{2002}{2} = 1001$