-5x的m+2n次方y的3m-n次方÷（-2x的3n次方y的2m+n次方）=5/2[x^(m+2n-3n)]*[y^(3m-n-2m-n)]=5/2(x^m-n)*(y^m-2n)根据题意有：m-n=3 m-2n=2解得，m=4 n=1∴m+n=4+1=5
