Question

Ma ajutati si pe mine la calculul 1+2+3....+20

Answer 1

$\frac{20(20+1)}{2}=10*21=210$

Answer 2

$\boxed{ \frac{n(n+1)}{2} } \\ \\ \text{n=ultimul numar din sir } \\ \\ 1+2+3+...+20= \frac{20(20+1)}{2} = \frac{20*21}{2}=10*21=\boxed{ \boxed{210 }}$