1/2*7+1/7*12+1/12*17+1/17*22
=1/5[1/2-1/7]+1/5[1/7-1/12]+1/5[1/12-1/17]+1/5[1/17-1/22]
=1/5[1/2-1/7+1/7-1/12+1/12-1/17+1/17-1/22]
=1/5[1/2-1/22]
=1/5*5/11
=1/11
两个数是互质数,最小公倍数是20,这两个数是（4）和（5）或（1）和（20）
20=4×5=1×20