a.
\(\dfrac{3}{x-3}-\dfrac{6x}{9-x^2}+\dfrac{x}{x+3}\) (đề thế này đúng ko? :D)
\(=\dfrac{3}{x-3}+\dfrac{6x}{\left(x-3\right)\left(x+3\right)}+\dfrac{x}{x+3}\)
\(=\dfrac{3\left(x+3\right)+6x+x\left(x-3\right)}{\left(x+3\right)\left(x-3\right)}=\dfrac{x^2+6x+9}{\left(x+3\right)\left(x-3\right)}=\dfrac{\left(x+3\right)^2}{\left(x+3\right)\left(x-3\right)}=\dfrac{x+3}{x-3}\)
b.
\(\left(x+2\right)^2-\left(x+1\right)\left(x-1\right)=\left(x^2+4x+4\right)-\left(x^2-1\right)=4x+6\)
c.
\(\dfrac{5x^2-10xy}{x^2-4xy+4y^2}\)
\(=\dfrac{5x\left(x-2y\right)}{x^2-2\cdot x\cdot2y+\left(2y\right)^2}=\dfrac{5x\left(x-2y\right)}{\left(x-2y\right)^2}=\dfrac{5x}{x-2y}\)