$\sqrt{4x-20}+3.\sqrt{ \dfrac{x-5}{9}}-\dfrac{1}{3}\sqrt{9x-45}=4$
`đkxđ:x>=5`
`<=>`$\sqrt{4(x-5)}+3.\dfrac{\sqrt{x-5}}{3}-\dfrac{1}{3}.\sqrt{9(x-5)}=4$
`<=>`$2\sqrt{x-5}+\sqrt{x-5}-\sqrt{x-5}=4$
`<=>`$\sqrt{x-5}=2$
` <=>x-5=4`
`<=>x=9(tm)`
vậy `x=9`
`b)`$\dfrac{2}{x^2-y^2}.\sqrt{\dfrac{3x^2+6xy+3y^2}{4}}$
$=\dfrac{2}{x^2-y^2}.\sqrt{\dfrac{3(x^2+2xy+y^2)}{2^2}}$
$=\sqrt{\dfrac{3(x+y)^2}{(x^2-y^2)^2} }$
$=\sqrt{\dfrac{3(x+y)^2}{(x-y)^2(x+y)^2}} $
$=\dfrac{\sqrt{3}}{x-y}$
