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x^{2}-8x=-4
All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. The quadratic formula gives two solutions, one when ± is addition and one when it is subtraction.
x^{2}-8x-\left(-4\right)=-4-\left(-4\right)
Add 4 to both sides of the equation.
x^{2}-8x-\left(-4\right)=0
Subtracting -4 from itself leaves 0.
x^{2}-8x+4=0
Subtract -4 from 0.
x=\frac{-\left(-8\right)±\sqrt{\left(-8\right)^{2}-4\times 4}}{2}
This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -8 for b, and 4 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.
x=\frac{-\left(-8\right)±\sqrt{64-4\times 4}}{2}
Square -8.
x=\frac{-\left(-8\right)±\sqrt{64-16}}{2}
Multiply -4 times 4.
x=\frac{-\left(-8\right)±\sqrt{48}}{2}
Add 64 to -16.
x=\frac{-\left(-8\right)±4\sqrt{3}}{2}
Take the square root of 48.
x=\frac{8±4\sqrt{3}}{2}
The opposite of -8 is 8.
x=\frac{4\sqrt{3}+8}{2}
Now solve the equation x=\frac{8±4\sqrt{3}}{2} when ± is plus. Add 8 to 4\sqrt{3}.
x=2\sqrt{3}+4
Divide 8+4\sqrt{3} by 2.
x=\frac{8-4\sqrt{3}}{2}
Now solve the equation x=\frac{8±4\sqrt{3}}{2} when ± is minus. Subtract 4\sqrt{3} from 8.
x=4-2\sqrt{3}
Divide 8-4\sqrt{3} by 2.
x=2\sqrt{3}+4 x=4-2\sqrt{3}
The equation is now solved.
x^{2}-8x=-4
Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.
x^{2}-8x+\left(-4\right)^{2}=-4+\left(-4\right)^{2}
Divide -8, the coefficient of the x term, by 2 to get -4. Then add the square of -4 to both sides of the equation. This step makes the left hand side of the equation a perfect square.
x^{2}-8x+16=-4+16
Square -4.
x^{2}-8x+16=12
Add -4 to 16.
\left(x-4\right)^{2}=12
Factor x^{2}-8x+16. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.
\sqrt{\left(x-4\right)^{2}}=\sqrt{12}
Take the square root of both sides of the equation.
x-4=2\sqrt{3} x-4=-2\sqrt{3}
Simplify.
x=2\sqrt{3}+4 x=4-2\sqrt{3}
Add 4 to both sides of the equation.