a: ĐKXĐ: \(x\notin\left\{2;3;-1;1\right\}\)
b: \(B=\dfrac{2}{x^2-5x+6}:\dfrac{x-1}{x+1}\)
\(=\dfrac{2}{\left(x-3\right)\left(x-2\right)}\cdot\dfrac{x+1}{x-1}\)
\(=\dfrac{2\left(x+1\right)}{\left(x-1\right)\left(x-2\right)\left(x-3\right)}\)
Thay x=10 vào B, ta được:
\(B=\dfrac{2\left(10+1\right)}{\left(10-1\right)\left(10-2\right)\left(10-3\right)}=\dfrac{2\cdot11}{9\cdot8\cdot7}=\dfrac{11}{252}\)
Thay x=20 vào B, ta được:
\(B=\dfrac{2\left(20+1\right)}{\left(20-1\right)\left(20-2\right)\left(20-3\right)}=\dfrac{2\cdot21}{19\cdot18\cdot17}=\dfrac{7}{969}\)
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