Question

Suma S=18+25+32+.........+151 este egala cu..

Answer 1

$\frac{n(n+1)}{2}$ - aplicam formula asta
151*152=22952
22952/2=11476

Answer 2

 S=18 + (18+1*7) + (18+2*7) + ... +(18+19*7)=
=(18+18+...+18)+7*(1+2+3+...+19)=
=18*20 + 7*(19*20:2)=360+1330 = 1690 ;