证明：原式=(x^5+3x^4y)-(5x^3y^2+15x^2y^3)+(4xy^4+12y^5)
=x^4(x+3y)-5x^2y^2(x+3y)+4y^4(x+3y)
=(x+3y)(x^4-5x^2y^2+4y^4)
=(x+3y)(x^2-4y^2)(x^2-y^2)
=(x+3y)(x+2y)(x-2y)(x+y)(x-y)．
当y=0时,原式=x^5,不等于33；当y不等于0时,x+3y,x+2y,x-2y,x+y,x-y互不相等,而33也不能分成四个以上不同因数的积,所以原命题成立.