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第85页

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解：$\frac {2}{a²+2a+1}= \frac {2(a-1)}{(a+1)^2(a-1)}，$

$\frac {a}{a²-1}=\frac {a(a+1)}{(a+1)^2(a-1)}$

解：$\frac {2}{9-4m²}= \frac {2(3-2m)}{(3-2m)^2(3+2m)}，$

$\frac {3m}{4m²-12m+9}=\frac {3m(3+2m)}{(3-2m)^2(3+2m)}$

解：$-\frac {1}{8x^4y} =-\frac {3y^2z^2}{24x^4y^3z^2}，$

$\frac {2}{3x²y^3z}=\frac {16x^2z}{24x^4y^3z^2}，$

$\frac {5}{6xz²}=\frac {20x^3y^3}{24x^4y^3z^2}$

解：$\frac {1}{x-2}= \frac {(x+3)^2}{(x-2)(x+3)^2}，$

$\frac {1}{(x-2)(x+3)}=\frac {x+3}{(x-2)(x+3)^2}，$

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

解：由$ab = 1，$得$b=\frac {1}{a}，$

把$b=\frac {1}{a}$代入$M，$$N，$

得$M=\frac {1}{1 + a}+\frac {a}{1 + a}，$$N=\frac {a}{1 + a}+\frac {1}{1 + a}，$

所以$M = N$