mn分之m-n等于n分之1减m分之1
(m-n)/mn=1/n-1/m
1*3分之4加3*5分之4加5*7分之四
=4(1/1*3+1/3*5+1/5*7)
=4(1/2(1-1/3)+1/2(1/3-1/5)+1/2(1/5-1/7))
=2(1-1/7)
=12/7