mx³ + 3nxy² - 2x³ - xy² + 2x + y
=(m-2)x³ +(3n - 1)xy² + 2x + y
不含三次项,所以 m-2=0,且3n - 1 = 0
所以,m = 2,n = 1/3
所以,-m² + 3n = - 2² + 3(1/3 )= -4 + 1 = -3
mx³ + 3nxy² - 2x³ - xy² + 2x + y
=(m-2)x³ +(3n - 1)xy² + 2x + y
不含三次项,所以 m-2=0,且3n - 1 = 0
所以,m = 2,n = 1/3
所以,-m² + 3n = - 2² + 3(1/3 )= -4 + 1 = -3