2m-3n/3m+5n=3 则：3m+5n/2m-3n=1/3
(4m-6n/18m+30n)+9m+15n/4m-6n
=2（2m+3n)/6(3m+5n)+3(3m+5n)/2(2m-3n)
=1/3 * 3 + 3/2 * 1/3
=1+1/2
=3/2