Bài 5 :
1) `x^(2)-4=0`
`⇔x^2=4`
`⇔x=±2`
2) `x^(2)-25=0`
`<=>x^2=25`
`<=>x=±5`
3) `x^(2)-36=0`
`⇔x^2=36`
`⇔x=±6`
4) `x^(2)-81=0`
`⇔x^(2)=81`
`⇔x=±9`
5) `x^(2)-49=0`
`<=>x^2=49`
`<=>x=±7`
6) `2x^(2)-8=0`
`⇔2(x^(2)-4)=0`
`⇔x^(2)=4`
`⇔x=±2`
7) `3x^(2)-75=0`
`⇔3(x^(2)-25)=0`
`⇔x^(2)=25`
`⇔x=±5`
8) `2x^(2)-72=0`
`⇔2(x^(2)-36)=0`
`⇔x^2=36`
`⇔x=±6`
9) `2x^(2)-98=0`
`⇔2(x^(2)-49)=0`
`⇔x^2=49`
`⇔x=±7`
10) `2x^(2)-162=0`
`⇔2(x^(2)-81)=0`
`⇔x^(2)=81`
`⇔x=±9`
11) `(x+3)^2=4`
$⇔\left[\begin{matrix}x+3=2\\x+3=-2\end{matrix}\right.$
$⇔\left[\begin{matrix}x=-1\\x=-5\end{matrix}\right.$
12) `(x+2)^(2)=25`
$⇔\left[\begin{matrix}x+2=5\\x+2=-5\end{matrix}\right.$
$⇔\left[\begin{matrix}x=3\\x=-7\end{matrix}\right.$
13) `(x-7)^2=36`
$⇔\left[\begin{matrix}x-7=6\\x-7=-6\end{matrix}\right.$
$⇔\left[\begin{matrix}x=13\\x=1\end{matrix}\right.$
14) `(x-1)^(2)-81=0`
`⇔(x-1)^2=81`
$⇔\left[\begin{matrix}x-1=9\\x-1=-9\end{matrix}\right.$
$⇔\left[\begin{matrix}x=10\\x=-8\end{matrix}\right.$
15) `x^(2)+6x+9=0`
`⇔(x+3)^2=0`
`⇔x+3=0`
`⇔x=-3`
16) `x^(2)+4x+4=0`
`⇔(x+2)^2=0`
`⇔x+2=0`
`⇔x=-2`
17) `x^(2)-14x+49=0`
`⇔(x-7)^2=0`
`⇔x-7=0`
`⇔x=7`
18) `x^(2)-2x+10=0`
(Phần này là sử dụng căn mà mình không giỏi phần đó nên xin lỗi bạn ạ)
19) `x^(2)+10x+25=0`
`⇔(x+5)^2=0`
`⇔x+5=0`
`⇔x=-5`
20) `x^(2)-12x+36=0`
`⇔(x-6)^2=0`
`⇔x-6=0`
`⇔x=6`
21) `x^(2)+6x+5=0`
`⇔x^(2)+x+5x+5=0`
`⇔x(x+1)+5(x+1)=0`
`⇔(x+1)(x+5)=0`
$⇔\left[\begin{matrix}x+1=0\\x+5=0\end{matrix}\right.$
$⇔\left[\begin{matrix}x=-1\\x=-5\end{matrix}\right.$
22) `x^(2)+4x-21=0`
`⇔x^(2)+7x-3x-21=0`
`⇔x(x+7)-3(x+7)=0`
`⇔(x+7)(x-3)=0`
$⇔\left[\begin{matrix}x+7=0\\x-3=0\end{matrix}\right.$
$⇔\left[\begin{matrix}x=-7\\x=3\end{matrix}\right.$
23) `x^(2)-14x+13=0`
`⇔x^(2)-x-13x+13=0`
`⇔x(x-1)-12(x-1)=0`
`⇔(x-1)(x-12)=0`
$⇔\left[\begin{matrix}x-1=0\\x-12=0\end{matrix}\right.$
$⇔\left[\begin{matrix}x=1\\x=12\end{matrix}\right.$
24) `x^(2)-2x-80=0`
`⇔x^(2)-10x+8x-80=0`
`⇔x(x-10)+8(x-10)=0`
`⇔(x-10)(x+8)=0`
$⇔\left[\begin{matrix}x-10=0\\x+8=0\end{matrix}\right.$
$⇔\left[\begin{matrix}x=10\\x=-8\end{matrix}\right.$
25) `x^(2)+10x+16=0`
`⇔x^(2)+8x+2x+16=0`
`⇔x(x+8)+2(x+8)=0`
`⇔(x+8)(x+2)=0`
$⇔\left[\begin{matrix}x+8=0\\x+2=0\end{matrix}\right.$
$⇔\left[\begin{matrix}x=-8\\x=-2\end{matrix}\right.$
26) `x^(2)-12x+11=0`
`⇔x^(2)-11x-x+11=0`
`⇔x(x-11)-(x-11)=0`
`⇔(x-11)(x-1)=0`
$⇔\left[\begin{matrix}x-11=0\\x-1=0\end{matrix}\right.$
$⇔\left[\begin{matrix}x=11\\x=1\end{matrix}\right.$
`#Study well`
