(81减x的4次方）除以（x的平方加9）除以（x加3
问题描述：
(81减x的4次方）除以（x的平方加9）除以（x加3
答
(81-x^4)/[(x^2+9)(x+3)]
=(9-x^2)(9+x^2)/[(x^2+9)(x+3)]
=(9-x^2)/(x+3)
=(3-x)(3+x)/(x+3)
=3-x