Đáp án: 81/10
Giải thích các bước giải:
1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90
= 1- 1/2+ 1- 1/6 + 1- 1/12 + 1- 1/20 + 1- 1/30 + 1- 1/42 + 1- 1/56 + 1- 1/72 + 1- 1/90
= 9- (1/2+ 1/6 +1/12 +1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 )
= 9- (1/1x2+ 1/2x3 +1/3x4 +1/4x5 + 1/5x6 + 1/6x7 + 1/7x8 + 1/8x9 + 1/9x10 )
= 9- ( 1/2+ 1/2 +1/3 +1/3 + 1/4 + 1/4 + 1/5 +1/5 + 1/6+1/6 +1/7+1/7 + 1/8 + 1/8 +1/9+1/9+1/10
= 9- ( 1-1/10 )
= 9- 9/10
=81/10
