楼上的答案都是没错,应该这样做:
1/1 - 1/5 = 4/(1*5)
1/5 - 1/9 = 4/(5*9)
.
所以:
4/(1*5) + 4/(5*9) + 4/(9*13) + 4/(13*17) + 4/(17*21)
=1-1/5 + 1/5-1/9 + 1/9-1/13 + 1/13-1/17 + 1/17-1/21
=1-1/21
=20/21
题目原式=3/4 * 20/21 = 5/7
楼上的答案都是没错,应该这样做:
1/1 - 1/5 = 4/(1*5)
1/5 - 1/9 = 4/(5*9)
.
所以:
4/(1*5) + 4/(5*9) + 4/(9*13) + 4/(13*17) + 4/(17*21)
=1-1/5 + 1/5-1/9 + 1/9-1/13 + 1/13-1/17 + 1/17-1/21
=1-1/21
=20/21
题目原式=3/4 * 20/21 = 5/7