原式=（x-3y）^4+x^2-2*x*3y+(3y)^2-2 =（x-3y）^4+(x-3y)^2-2 =（x-3y）^4+2*1/2*(x-3y)^2+(1/2)^2-(1/2)^2-2 =[(x-3y)+1/2]^2-9/4 =(x-3y+1/2)^2-9/4=(x-3y+1/2+3/2)(x-3y+1/2-3/2)=(x-3y+2)(x-3y-1)