先化简.再求值,x2-2x/x2-1 ÷(x-1-2x-1/x+2),其中x=1/2
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解：
原式={x(x-2)/[(x+1)(x-1)]}÷[(x-1)(x+1)/(x-1)-(2x-1)/(x+1)]
={x(x-2)/[(x+1)(x-1)]}÷[(x²-1-2x+1)/(x+1)]
={x(x-2)/[(x+1)(x-1)]}÷[(x²-2x)/(x+1)]
={x(x-2)/[(x+1)(x-1)]}×{(x+1)/[x(x-2)]}
=1/(x-1)
当x=1/2时
原式=1/(1/2-1)
=1/(-1/2)
=-2
原式={x(x-2)/[(x+1)(x-1)]}÷[(x-1)(x+1)/(x-1)-(2x-1)/(x+1)]
={x(x-2)/[(x+1)(x-1)]}÷[(x²-1-2x+1)/(x+1)]
={x(x-2)/[(x+1)(x-1)]}÷[(x²-2x)/(x+1)]
={x(x-2)/[(x+1)(x-1)]}×{(x+1)/[x(x-2)]}
=1/(x-1)
当x=1/2时
原式=1/(1/2-1)
=1/(-1/2)
=-2
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