x^2y/5-x^4-y^2/100
=-1/100(-20x^2+100x^4+y^2)
=-1/100(100x^4-20x^2+y^2)
=-1/100(10x^2-y)^2不是x^2y/5！！是1/5乘以x的平方yx^2y/5就是1/5乘以x的平方y