3x2y3和-5x2+2m+ny3是同类项,那么:
2+2m+n=2,
2m+n=0,
n=-2m;
把n=-2m代入
3m3−m2n+3mn2+9n3
5m3+3m2n−6mn2+9n3中可得:
3m3−m2n+3mn2+9n3
5m3+3m2n−6mn2+9n3,
=
3m3−m2(−2m)+3m(−2m)2+9(−2m)3
5m3+3m2(−2m)−6m(−2m)2+9(−2m)3,
=
3m3+2m3+12m3−72m3
5m3−6m3−24m3−72m3
=
−55m3
−97m3
=
55
97;
故答案为:
55
97.