Question

Calculati sumele :7+14+21+...+3157=

Answer 1

S=7x1+7x2+7x3+...+7x451=
S=7x(1+2+3+...+451)
S=451x452:2=451x226
S=101.926

Answer 2

$\displaystyle 7+14+21+...+3157=7(1+2+3+...+451)= \\ \\ =7 \times \frac{451(451+1)}{2} =7 \times \frac{451 \times 452}{2} =7 \times \frac{203852}{2} = \\ \\ =7 \times 101926=713482$