`a)9-(x+5)^2=0`
`<=>(3-x-5)(3+x+5)=0`
`<=>(-2-x)(x+8)=0`
`<=>[(x=-2),(x=-8):}`
Vậy `x\in{-2;-8}`
`b)(x-5)^2-(3x+1)^2=0`
`<=>(x-5-3x-1)(x-5+3x+1)=0`
`<=>(-2x-6)(4x-4)=0`
`<=>[(-2x=6),(4x=4):}`
`<=>[(x=-3),(x=1):}`
Vậy `x\in{-3;1}`
`c)(2x-1)^2+(x+3)^2-5(x-7)(x+7)=0`
`<=>4x^2-4x+1+x^2+6x+9-5(x^2-49)=0`
`<=>5x^2+2x+10-5x^2+245=0`
`<=>2x=-10-245`
`<=>x=\frac{-255}{2}`
Vậy `x=-255/2`