[(1/2)/(4/5)]/[（2/9)/(3/8）]=[(1/2)*(5/4)]/[（2/9)*(3/8）]
=[(5/8)/(1/8)]/[（2/9)*3]=5/(2/3)=5*3/2
=15/2
[(5/6)-(3/4)]/[(5/12)/(9/5)]=(1/12)*(12/5)*(9/5)
=9/25
(3/7)/【5/7-（3/7+1/5）】=(3/7)/[(5/7-3/7)-1/5]
=(3/7)/(2/7+1/5)=(3/7)/(2*5/5*7-1*7/5*7)=(3/7)*(7*5)/3
=5