giúp mik bài3B 4A vs ạ
3B.
a.
\(\dfrac{6}{7-x}+\dfrac{-4}{x}+\dfrac{6}{x-7}+\dfrac{2}{x-2}+\dfrac{4}{x}+\dfrac{-5}{x+2}\)
\(=\dfrac{6}{7-x}-\dfrac{6}{7-x}+\dfrac{-4+4}{x}+\dfrac{2\left(x+2\right)}{\left(x-2\right)\left(x+2\right)}+\dfrac{-5\left(x-2\right)}{\left(x-2\right)\left(x+2\right)}\)
\(=0+0+\dfrac{2x+4-5x+10}{\left(x-2\right)\left(x+2\right)}\)
\(=\dfrac{-3x+14}{\left(x-2\right)\left(x+2\right)}\)
b.
\(\dfrac{x-1}{x^2-4}-\dfrac{1}{x+2}+\dfrac{x+2}{x}+\dfrac{1}{x+2}+\dfrac{x+2}{-x}-\dfrac{3}{x-2}\)
\(=\dfrac{x-1}{\left(x-2\right)\left(x+2\right)}-\dfrac{3\left(x+2\right)}{\left(x-2\right)\left(x+2\right)}+\left(\dfrac{1}{x+2}-\dfrac{1}{x+2}\right)+\left(\dfrac{x+2}{x}-\dfrac{x+2}{x}\right)\)
\(=\dfrac{x-1-3x-6}{\left(x-2\right)\left(x+2\right)}+0+0\)
\(=\dfrac{-2x-7}{\left(x-2\right)\left(x+2\right)}\)
4A.
\(\dfrac{3x+1}{\left(x-1\right)^2}-\dfrac{1}{x+1}+\dfrac{x+3}{1-x^2}\)
\(=\dfrac{\left(3x+1\right)\left(x+1\right)}{\left(x+1\right)\left(x-1\right)^2}-\dfrac{\left(x-1\right)^2}{\left(x+1\right)\left(x-1\right)^2}-\dfrac{\left(x+3\right)\left(x-1\right)}{\left(x+1\right)\left(x-1\right)^2}\)
\(=\dfrac{3x^2+4x+1-\left(x^2-2x+1\right)-\left(x^2+2x-3\right)}{\left(x+1\right)\left(x-1\right)^2}\)
\(=\dfrac{x^2+4x+3}{\left(x+1\right)\left(x-1\right)^2}=\dfrac{\left(x+1\right)\left(x+3\right)}{\left(x+1\right)\left(x-1\right)^2}\)
\(=\dfrac{x+3}{\left(x-1\right)^2}\)
b.
\(\dfrac{2x}{x^2-4}-\left(\dfrac{2}{x+5}+\dfrac{3-x}{x+1}\right)+\left(\dfrac{2}{x+5}-\left(\dfrac{4}{x^2-4}+\dfrac{x-3}{x+1}\right)\right)\)
\(=\dfrac{2x}{x^2-4}+\left(\dfrac{2}{x+5}-\dfrac{2}{x+5}\right)+\left(\dfrac{x-3}{x+1}-\dfrac{x-3}{x+1}\right)-\dfrac{4}{x^2-4}\)
\(=\dfrac{2x-4}{x^2-4}=\dfrac{2\left(x-2\right)}{\left(x-2\right)\left(x+2\right)}\)
\(=\dfrac{2}{x+2}\)
AI GIÚP MIK VS Ạ. MAI MIK IK HỌC R Ạ 
