19/20+29/30+41/42+55/56+71/72
=1- 1/20 +1 - 1/30 +1 -1/42 +1 -1/56 +1- 1/72
=5- (1/20 + 1/30 + 1/42 + 1/56 + 1/72）
=5-(1/4 - 1/5 +1/5 - 1/6+ 1/6 - 1/7 +1/7 -1/8 +1/8 -1/9)
=5 - （1/4 - 1/9)
=5 - 5/36
=4又31/36