Đáp án:
Giải thích các bước giải:
`f)`
`x^2 + 1 = 81`
`-> x^2 = 81`
`-> x = \pmsqrt{81}`
`-> x = \pm9`
Vậy `x \in {-9,9}`
`g)`
`2x^2 = 6`
`-> x^2 = 3`
`-> x = \pmsqrt3`
Vậy `x \in {-sqrt3,sqrt3}`
`h)`
`7x^2 = 91`
`-> x^2 = 13`
`-> x = \pmsqrt{13}`
Vậy `x \in {-sqrt{13},sqrt{13}}`
`i)`
`(x-1)^2=9`
`-> |x-1| = 3`
`->`\(\left[ \begin{array}{l}x-1=3\\x-1=-3\end{array} \right.\)
`->`\(\left[ \begin{array}{l}x=4\\x=-2\end{array} \right.\)
Vậy `x \in {-2,4}`
`k)`
`(2x+3)^2 = 25`
`-> |2x+3| = 5`
`->`\(\left[ \begin{array}{l}2x+3=5\\2x+3=-5\end{array} \right.\)
`->`\(\left[ \begin{array}{l}2x=2\\2x=-8\end{array} \right.\)
`->`\(\left[ \begin{array}{l}x=1\\x=-4\end{array} \right.\)
Vậy `S = {-4,1}`
