Question

1乘2分之一加2分1乘3分之一加3分之1乘4分之1~~~~6分之1乘7分之1等于多少

Answer 1

1/2+1/6+1/12+1/20+1/30+1/42
=6/7

Answer 2

1×1/2=1-1/2
1/2×1/3=1/2-1/3
1/3×1/4=1/3-1/4
.
.
1/6×1/7=1/6-1/7

原式=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7