:1/a+1-a+2/a²-1·a²-2a+1/a²+4a+4
=1/(a+1)-(a+2)/(a+1)(a-1)×(a-1)²/(a+2)²
=1/(a+1)-(a-1)/(a+1)(a+2)
=(a+2-a+1)/(a+1)(a+2)
=3/(a+1)(a+2);
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