（a-a/a+1)/(a*a-2a)*(a+1)/(a*a+3a+2)
=a²/(a+1)/[a(a-2)]*(a+1)/[(a+1)(a+2)]
=a/[(a+1)(a-2)]×1/(a+2)
=a/[(a+1)(a-2)(a+2)]
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