Question

tính tổng 1/2 x ( 1+ 2) + 1/3x( 1+ 2+3) + 1/4 x ( 1+ 2+ 3+ 4) + ...+ 1/100x (n 1+ 2+ 3+ 4+ 5+ ....+ 100)

Answer 1

\(\frac{1}{2}.\left(1+2\right)+\frac{1}{3}.\left(1+2+3\right)+...+\frac{1}{100}\left(1+2+...+100\right)\)

\(=\frac{1+2}{2}+\frac{1+2+3}{3}+...+\frac{1+2+...+100}{100}\)

\(=\frac{\left(1+2\right).2:2}{2}+\frac{\left(1+2+3\right).3:2}{3}+...+\frac{\left(1+2+...+100\right).100:2}{100}\)

\(=\left(1+2\right):2+\left(1+2+3\right):2+....\left(1+2+...+100\right):2\)

\(=\left[\left(1+2\right)+\left(1+2+3\right)+...+\left(1+2+...+100\right)\right]:2\)

\(=\left(100.1+99.2+....+1.100\right):2=171700:2=85850\)

Nếu không hiểu cái trong ngoặc tính sao thì báo tớ ;)