Solve x^2/4-10x+16=0

$$\frac { x ^ { 2 } } { 4 } - 10 x + 16 = 0$$

Solve for x

$$x=4\sqrt{21}+20\approx 38.33030278$$

$$x=20-4\sqrt{21}\approx 1.66969722$$

Steps Using the Quadratic Formula

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.

$$\frac{1}{4}x^{2}-10x+16=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute \frac{1}{4} for a, -10 for b, and 16 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-10\right)±\sqrt{\left(-10\right)^{2}-4\times \frac{1}{4}\times 16}}{2\times \frac{1}{4}}$$

Square -10.

$$x=\frac{-\left(-10\right)±\sqrt{100-4\times \frac{1}{4}\times 16}}{2\times \frac{1}{4}}$$

Multiply -4 times \frac{1}{4}.

$$x=\frac{-\left(-10\right)±\sqrt{100-16}}{2\times \frac{1}{4}}$$

Add 100 to -16.

$$x=\frac{-\left(-10\right)±\sqrt{84}}{2\times \frac{1}{4}}$$

Take the square root of 84.

$$x=\frac{-\left(-10\right)±2\sqrt{21}}{2\times \frac{1}{4}}$$

The opposite of -10 is 10.

$$x=\frac{10±2\sqrt{21}}{2\times \frac{1}{4}}$$

Multiply 2 times \frac{1}{4}.

$$x=\frac{10±2\sqrt{21}}{\frac{1}{2}}$$

Now solve the equation x=\frac{10±2\sqrt{21}}{\frac{1}{2}} when ± is plus. Add 10 to 2\sqrt{21}.

$$x=\frac{2\sqrt{21}+10}{\frac{1}{2}}$$

Divide 10+2\sqrt{21} by \frac{1}{2} by multiplying 10+2\sqrt{21} by the reciprocal of \frac{1}{2}.

$$x=4\sqrt{21}+20$$

Now solve the equation x=\frac{10±2\sqrt{21}}{\frac{1}{2}} when ± is minus. Subtract 2\sqrt{21} from 10.

$$x=\frac{10-2\sqrt{21}}{\frac{1}{2}}$$

Divide 10-2\sqrt{21} by \frac{1}{2} by multiplying 10-2\sqrt{21} by the reciprocal of \frac{1}{2}.

$$x=20-4\sqrt{21}$$

The equation is now solved.

$$x=4\sqrt{21}+20 x=20-4\sqrt{21}$$

Steps for Completing the Square

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.

$$\frac{1}{4}x^{2}-10x+16=0$$

Subtract 16 from both sides of the equation.

$$\frac{1}{4}x^{2}-10x+16-16=-16$$

Subtracting 16 from itself leaves 0.

$$\frac{1}{4}x^{2}-10x=-16$$

Multiply both sides by 4.

$$\frac{\frac{1}{4}x^{2}-10x}{\frac{1}{4}}=-\frac{16}{\frac{1}{4}}$$

Dividing by \frac{1}{4} undoes the multiplication by \frac{1}{4}.

$$x^{2}+\left(-\frac{10}{\frac{1}{4}}\right)x=-\frac{16}{\frac{1}{4}}$$

Divide -10 by \frac{1}{4} by multiplying -10 by the reciprocal of \frac{1}{4}.

$$x^{2}-40x=-\frac{16}{\frac{1}{4}}$$

Divide -16 by \frac{1}{4} by multiplying -16 by the reciprocal of \frac{1}{4}.

$$x^{2}-40x=-64$$

Divide -40, the coefficient of the x term, by 2 to get -20. Then add the square of -20 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-40x+\left(-20\right)^{2}=-64+\left(-20\right)^{2}$$

Square -20.

$$x^{2}-40x+400=-64+400$$

Add -64 to 400.

$$x^{2}-40x+400=336$$

Factor x^{2}-40x+400. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-20\right)^{2}=336$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-20\right)^{2}}=\sqrt{336}$$

Simplify.

$$x-20=4\sqrt{21} x-20=-4\sqrt{21}$$

Add 20 to both sides of the equation.

$$x=4\sqrt{21}+20 x=20-4\sqrt{21}$$