Question

efectuati(3+6+9+...................+30)-(2+4+6+........................+20)

Answer 1

(3+6+9+...+30) - (2+4+6+...+20) = 
= 3*(1+2+3+...+10) - 2*(1+2+3+...+10) =
= 3*55 - 2*55 =
= 165 - 110 =
= 55

Answer 2

3(1+2+3+...+10) - 2(1+2+3+ ... 10) = 1(1+2+3+...+10) =10*11 / 2 =55