Question

1/2+1/6+1/12+1/20+...+1/90. Tính nhanh chình bày luôn nha.

Answer 1

= 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + ...... + 1/9x10

= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4-1/5 +.......+ 1/9 -1/10

= 1/1 - 1/10

= 9/10

Answer 2

=1/1x2+1/2x3+1/3x4+1/4x5+......+1/9x10