Question

要使多项式mx^3+3nxy^2+2x^3-xy^2+y不含三次项,求m+3n是多少

Answer 1

先对这个式子合并同类项,可得：（m+2)x^3 + (3n-1) xy^2 + y ,要使该多项式不含三次项,即三次项的系数为零,则m+2=0 ,3n-1=0 ,可得：m = -2 ,3n = 1 ,所以m+3n = (-2)+1 = -1