Question

sa se calculeze suma 2+4+6+8...+20

Answer 1

modul 1:
2+4+6+8+....+20=2*(1+2+3+...+10)=2*[10*(10+1)]/2=10*11=110
modul 2:
2+4+6+8+.....+20=(20+2)*(numarul de termeni)/2
numarul de termeni=(20-2)/2+1=18/2+1=9+1=10
(20+2)*10/2=22*5=110

Answer 2

$\displaystyle 2+4+6+8+...+20=2(1+2+3+...+10)=2 \cdot \frac{10(10+1)}{2} = \\ \\ =2 \cdot \frac{10 \cdot 11}{2} =2 \cdot \frac{110}{2} =2 \cdot 55=110$