Question

am nevoie de ajutor!
2+4+6+8+10+12+14+16........+40

Answer 1

$\displaystyle 2+4+6+8+10+12+14+16+...+40=2(1+2+3+...+20)= \\ \\ =2 \times \frac{20(20+1)}{2} =2 \times \frac{20 \times 21}{2} =\not2 \times \frac{420}{\not2} =420$