`a) 25sqrt{(a-3)/25}-7sqrt{(4a-12)/9}-7sqrt{a^2-9}+18sqrt{(9a^2-81)/81}=0` ĐK:`a>=3`
`<=>25. 1/5sqrt{a-3}-7. 2/3sqrt{a-3}-7sqrt{a^2-9}+18. 1/3sqrt{a^2-9}=0`
`<=> 5sqrt{a-3}-14/3sqrt{a-3}-7sqrt{a^2-9}+6sqrt{a^2-9}=0`
`<=> 1/3sqrt{a-3}-sqrt{a^2-9}=0`
`<=> sqrt{a-3}(1/3-sqrt{a+3})=0`
`<=>`\(\left[ \begin{array}{l}a-3=0\\\sqrt{a+3}=\dfrac{1}{3}\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}a=3\\a+3=\dfrac{1}{9}\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}a=3(\text{tm})\\a=\dfrac{-26}{9} (\text{ktm})\end{array} \right.\)
Vậy `S={3}`
`b) sqrt{18x+9}-sqrt{8x+4}+1/3sqrt{2x+1}=4` ĐK:`x>=-1/2`
`<=> 3sqrt{2x+1}-2sqrt{2x+1}+1/3sqrt{2x+1}=4`
`<=> 4/3sqrt{2x+1}=4`
`<=> sqrt{2x+1}=3`
`<=> 2x+1=9`
`<=> 2x=8`
`<=> x=4 (\text{tm})`
Vậy `S={4}`
