Question

Calculati :
1. 2+4+6+8+10+......+20
2. 5+10+15+....+625

Answer 1

2*(1+2+3+...+10) 2*(10*11):2= 110 5*(1+2+3+...+125) 5*(125*126):2 5*7875= 39375

Answer 2

$1).2+4+6+8+10+...+20=2(1+2+3+...+10)= \\ =2* \frac{10(10+1)}{2} =10*11=110 \\ 2).5+10+15+...+625=5(1+2+3+...+125)= \\ =5* \frac{125(125+1)}{2} =5* \frac{125*126}{2} =5* \frac{15750}{2} =5*7875=39375$