1×4/1+4×7/1+7×10/1……+22×25/1+25×28/1=?
问题描述：
1×4/1+4×7/1+7×10/1……+22×25/1+25×28/1=?
答
1×4/1+4×7/1+7×10/1……+22×25/1+25×28/1=
=(1/3)[(1/1-1/4)+(1/4-1/7)+(1/7-1/10)+.+(1/22-1/25)+(1/25-1/28)]
=(1/3)(1-1/28)
=(1/3)*(27/28)
=9/28