若a=1/2，求（a^2-4/a^-4a+4-1/a-2）÷a+1/a+2的值
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(a^2-4)/(a^2-4a+4)-1/(a-2)/(a-1)/(a+2)
=[(a-2)(a+2)]/(a-2)^2-(a+2)/[(a-2)(a-1)]
=(a+2)/(a-2)-(a+2)/[(a-2)(a-1)]
=(a+2)/(a-2)*[1-1/(a-1)]
=(a+2)/(a-2)*[(a-1-1)/(a-1)]………………此处通分
=(a+2)/(a-2)*[(a-2)/(a-1)]
=(a+2)/(a-1)
=(1/2+2)/(1/2-1)
=-5
=[(a-2)(a+2)]/(a-2)^2-(a+2)/[(a-2)(a-1)]
=(a+2)/(a-2)-(a+2)/[(a-2)(a-1)]
=(a+2)/(a-2)*[1-1/(a-1)]
=(a+2)/(a-2)*[(a-1-1)/(a-1)]………………此处通分
=(a+2)/(a-2)*[(a-2)/(a-1)]
=(a+2)/(a-1)
=(1/2+2)/(1/2-1)
=-5
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