化简求值：【a-2/（a2+2a）-a-1/（a2+4a+4】÷a-4/a+2，其中a=-12
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[(a-2)/(a^2+2a)-(a-1)/(a^2+4a+4)]÷(a-4)/(a+2)
=[(a-2)/a(a+2)-(a-1)/(a+2)^2]÷(a-4)/(a+2)
=[(a-2)/a(a+2)-(a-1)/(a+2)^2]*(a+2)/(a-4)
=(a-2)/a(a+2)*(a+2)/(a-4)-(a-1)/(a+2)^2*(a+2)/(a-4)
=(a-2)/a(a-4)-(a-1)/(a+2)(a-4)
=(a-2)(a+2)/a(a+2)(a-4)-a(a-1)/a(a+2)(a-4)
=[(a-2)(a+2)-a(a-1)]/a(a+2)(a-4)
=(a^2-4-a^2+a)/a(a+2)(a-4)
=(a-4)/a(a+2)(a-4)
=1/a(a+2)
=1/[-12*(-12+2)]
=1/120
=[(a-2)/a(a+2)-(a-1)/(a+2)^2]÷(a-4)/(a+2)
=[(a-2)/a(a+2)-(a-1)/(a+2)^2]*(a+2)/(a-4)
=(a-2)/a(a+2)*(a+2)/(a-4)-(a-1)/(a+2)^2*(a+2)/(a-4)
=(a-2)/a(a-4)-(a-1)/(a+2)(a-4)
=(a-2)(a+2)/a(a+2)(a-4)-a(a-1)/a(a+2)(a-4)
=[(a-2)(a+2)-a(a-1)]/a(a+2)(a-4)
=(a^2-4-a^2+a)/a(a+2)(a-4)
=(a-4)/a(a+2)(a-4)
=1/a(a+2)
=1/[-12*(-12+2)]
=1/120
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