Chứng minh các đẳng thức sau:

a, \(\frac{3x}{x+y}=\frac{-3x\left(x-y\right)}{y^2-x^2}\left(x\ne-y,x\ne y\right)\)

b, \(\frac{x-2}{-x}=\frac{8xy^2}{12ay}\left(a\ne0,y\ne0\right)\)

c, \(\frac{x+y}{3a}=\frac{3a\left(x+y\right)^2}{9a^2\left(x+y\right)}\left(a\ne0,x\ne-y\right)\)