Question

(2+4+6+ ....+20)-(1+3+5 ...+19)
Ajutor va rogg

Answer 1

1+2+..+n=n(n+1):2
1+3+5+..+2n-1=n²   2n-1=19=>n=10
..................................

2(1+2+..+10) - (1+3+...+19) = 
= 2·10·11:2 - 10²
= 110 - 100
= 10