应用了立方和的公式：
a³+b³=(a+b)(a²-ab+b²)
[1/(x+2)]-[12/(x3+8)]
=[1/(x+2)]-[12/(x3+23)]
=[1/(x+2)]-[12/(x+2)(x2-2x+22)]
=[1/(x+2)]-[12/(x+2)(x2-2x+4)]
=[(x2-2x+4)/(x+2)(x2-2x+4)]
-[12/(x+2)(x2-2x+4)]
=(x2-2x+4-12)/(x+2)(x2-2x+4)
=(x2-2x-8)/(x+2)(x2-2x+4)
=(x+2)(x-4)/(x+2)(x2-2x+4)
=(x-4)/(x2-2x+4)
最简单的通分后相加
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