Completing the Square Question?

Question

On one question like x^2-2x-2=0 I simply divide b by 2
but on another like x^2+8x-3=0
I have to take the coefficient of b and add to both sides etc
How do I know when to do this or not?

Answer 1

x^2-2x-2=0

x^2-2x=2

Add 1 both sides for complete squares.

x^2-2x+1=2+1

(x-1)^2=3

x-1= ± sqrt(3)

x=1± sqrt(3)

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x^2+8x-3=0

x^2+8x=3

Add 16 both sides for complete squares.

x^2+8x+16=3+16

(x+4)^2=19

x+4= ± sqrt(19)

x=-4± sqrt(19)