Question

s=2+4+6+.............+30=?
repedeeeeeeeeeeeee​

Answer 1

$S=2 + 4 + 6 + ... + 30 \\ S= 2(1 + 2 + 3 + ... + 15) \\ S= 2 \times \frac{15 \times 16}{2} \\ S= 2 \times 15 \times 8 = 240$

îl dăm pe 2 factor comun, apoi folosim formula lui Gauss

$1 + 2 + 3 + ... + x = \frac{x \times (x + 1)}{2}$