先化简,再求值:(a-1/(x^2-4a+4)-a+2/(a^2-2a))÷（（4/a）-1）,其中

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(a-1/(a^2-4a+4)-a+2/(a^2-2a))÷（（4/a）-1）
=[(a-1)/(a^2-4a+4)-(a+2)/(a^2-2a)]÷[(4/a)-1]
=[(a-1)/(a-2)²-(a+2)/a(a-2)]÷[(4-a)/a]
={[a(a-1)-(a+2)(a-2)]/[a(a-2)²]}÷[(4-a)/a]
={[a²-a-a²+4]/[a(a-2)²]}÷[(4-a)/a]
={[4-a]/[a(a-2)²]}*[a/(4-a)]
=1/(a-2)²
=1/[2√3-2)]²
=1/[4(√3-1)²]
=1/[4(4-2√3)]
=1/[8(2-√3)]
=(2+√3)/8