Question

1/5*8+1/8*11+1/11*14+1/14*17+1/17*20=?怎样算

Answer 1

1/(5*8)=(1/3)*(1/5-1/8)

1/(8*11)=(1/3)*(1/8-1/11)

其他项同理

原式=(1/3)*(1/5-1/8+1/8-1/11+...+1/17-1/20)

=(1/3)*(1/5-1/20)

=1/20