a^(m+1)*b^(n+2)*a^(2m)*b^(2n-1)=a^4*b^7
a^(m+1+2m)*b^(n+2+2n-1)=a^4*b^7
所以有
m+1+2m=4
n+2+2n-1=7
化简得
3m+1=4
3n+1=7
解得m=1,n=2
m+n=1+2=3