1又12分之1+2又20分之1+3又30分之1+4又42分之1
=(1+2+3+4)+(1/12+1/20+1/30+1/42)
=10+(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7)
=10+(1/3-1/7)
=10又4/21