Question

Biết 1²+2²+...+10²+20²=2870

Tính 1+(1+2)+(1+2+3)+...+(1+2+3+...+20)

Answer 1

= \(\frac{20.21:2+2870}{2}=\frac{210+2870}{2}=\frac{3080}{2}=1540\)

Answer 2

\(=\frac{1\left(1+1\right)}{2}+\frac{2\left(2+1\right)}{2}+\frac{3\left(3+1\right)}{2}+...+\frac{20\left(20+1\right)}{2}\)

\(=\frac{1+1+2.2+2+3.3+3+...+20.20+20}{2}\)

\(=\frac{\left(1+...+20\right)+\left(1.1+2.2+3.3+...+20.20\right)}{2}\)

Tính tiếp đi 

Answer 3

Thế mà không cho trước