若多项式mx³+3nxy²-2x³-xy²+y中不含三次项,求2m+3n的值

若多项式mx³+3nxy²-2x³-xy²+y中不含三次项,求2m+3n的值
若多项式mx³+3nxy²-2x³-xy²+y中不含三次项,求2m+3n的值

若多项式mx³+3nxy²-2x³-xy²+y中不含三次项,求2m+3n的值
mx³+3nxy²-2x³-xy²+y
＝（mx³-2x³）+(3nxy²-xy²)+y
=(m-2)x³+(3n-1)xy²+y
不含三次项,则有
m-2=0,3n-1=0
∴m=2,n=1/3
∴2m+3n
=2×2+3×(1/3)
=4+1
=5

mx³+3nxy²-2x³-xy²+y
=x³(m-2)+xy²(3n-1)+y
m-2=0
3n-1=0
m=2
n=1/3