(3/4+2/5)*(2/3-2/9)=(3/4+2/5)*(6/9-2/9) = (3/4+2/5)*(4/9) =(3/4)(4/9)+(2/5)(4/9)=1/3+8/45=15/45+8/45=23/45(1/3+1/5)/(2/3+2/5)=(1/3+1/5)/[2(1/3+1/5)]=1/2
(3/4+2/5)*(2/3-2/9)=23/20×4/9=23/45 (1/3+1/5)/(2/3+2/5)=8/15÷16/15=1/2
就通分算喽