若关于x,y的多项式mx³+3nxy²+2x²+2x³-xy²+y中不含三次项,
=(m+2)x³+(3n-1)xy²+2x²+y
(m+2)=0 (3n-1)=0
m=-2 n=1/3
m+3n的值
=-2+3*1/3
=-1