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第61頁(yè)

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C

$=\frac {a-b}{a-b}?$

$?=1$

$?=\frac {b(a+b)-ab}{(a+b)(a-b)}?$

$? =\frac {b^2}{a^2-b^2}?$

$?=\frac {(a+2)(2-a)-4}{2-a}?$

$? =\frac {a^2}{a-2}?$

$解：?(1)n ·\frac n{n+1}=n-\frac {n}{n+1}?$

$? (2)?右邊?=\frac {n(n+1)-n}{n+1}=\frac {n^2+n-n}{n+1}=\frac {n^2}{n+1}=n ·\frac n{n+1}=?左邊$

∴等式正確

A

$\frac {5}{3}$

2

-1

$? =\frac {2x-3}{x^3}?$

$?=\frac {a^2-b^2}{(a-b)^2}?$

$?=\frac {a+b}{a-b}$

$=\frac {2(x+3)+(1-x)(x-3)-6×2}{2(x+3)(x-3)}?$

$?=\frac {2x+6+x-3-x^2+3x-12}{2(x+3)(x-3)}?$

$?=\frac {-x^2+6x-9}{2(x+3)(x-3)}?$

$?=-\frac {x-3}{2x+6}$

$?=\frac {1-x+1+x}{1-x^2}+\frac 2{1+x^2}+\frac 4{1+x^4}?$

$? =\frac {2(1+x^2)+2(1-x^2)}{1-x^4}+\frac 4{1+x^4}?$

$? =\frac {4(1+x^4)+4(1-x^4)}{1-x^8}?$

$? =\frac 8{1-x^8}?$

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