Question

1/1x4+1/4x7+……+1/91x94=?

Answer 1

1/1x4=(1/3)*(1/1-1/4)

1/4*7=(1/3)(1/4-1/7)

..

1/91*94=(1/3)(1/91-1/94)

所以：

1/1x4+1/4x7+……+1/91x94

＝（1/3)*(1/1-1/4+1/4-1/7+..+1/91-1/94)

=(1/3)(1-1/94)

=31/94