Đáp án:
`a. sqrt{1-x}+sqrt{4-4x}-12=0`(Đk: `x\e1`)
`<=> sqrt{1-x}+2sqrt{1-x}-12=0`
`<=> 3sqrt{1-x}-12=0`
`<=> 3sqrt{1-x}=12`
`<=> sqrt{1-x}=4`
`<=> 1-x=16`
`<=> x=-15` (tm)
Vậy `S={-15}`
`b. sqrt{4x^2-4x+1}=3`
`<=> sqrt{(2x-1)^2}=3`
`<=> |2x-1|=3`
`<=> 2x-1=+-3`
`<=>`\(\left[ \begin{array}{l}2x-1=3 \ (x\ge \dfrac{1}{2})\\2x-1=-3 \ (x<\dfrac{1}{2})\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=2 \ (tm)\\x=-1 \ (tm)\end{array} \right.\)
Vậy `S={2;-1}`