Question

aceste 2 exerciți 2+4+6+......+80=

7+14+21.....+700=
repede dau coroana

Answer 1

2+4+6+...+80=2(1+2+3+...+40)=2x40x41:2=40x41=1640
deoarece am aplicat suma lui Gauss care zice ca 1+2+3+4+5+6+...+n=nx(n+1):2

7+14+21+...+700=7x(1+2+3+...+100)=7x100x101:2=35350