先化简，在求值：a^2-1?a-1-根号a^2-2a+1?a^2-a,其中a=1?2+根号3

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分母有理化
a=2-√3
则a-1<0
|a-1|=-(a-1)
且1/a=2+√3

原式=(a+1)(a-1)/(a-1)-√(a-1)²/a(a-1)
=a+1-|a-1|/a(a-1)
=a+1+1/a
=2-√3+1+2+√3
=5

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