Question

1+2+3+....+80 calculato suma

Answer 1

80x(80+1):2=80x81:2=40x81  si calculeaza si tu 40x81

Answer 2

$\boxed{ \frac{n(n+1)}{2} } \\ \text{Aplici formula asta , unde n e ultimul numar din sirul respectiv } \\ \\ \frac{80(80+1)}{2}= \frac{80*81}{2}=40*81=3240$