(x^2-9)(x^2+9)-(9-x^2)^2
问题描述：
(x^2-9)(x^2+9)-(9-x^2)^2
答
(x²-9)(x²+9)-(9-x²)²
＝(x²-9)(x²+9)-(x²-9) ²
=(x²-9)[x²+9-(x²-9)]
=(x²-9)(x²+9-x²+9)
=18(x²-9)
=18(x+3)(x-3)