Solve x^2-8x+16=121

$$x ^ { 2 } - 8 x + 16 = 121$$

Solve for x

$$x=-7$$

$$x=15$$

Steps Using Factoring

Subtract 121 from both sides.

$$x^{2}-8x+16-121=0$$

Subtract 121 from 16 to get -105.

$$x^{2}-8x-105=0$$

To solve the equation, factor x^{2}-8x-105 using formula x^{2}+\left(a+b\right)x+ab=\left(x+a\right)\left(x+b\right). To find a and b, set up a system to be solved.

$$a+b=-8 ab=-105$$

Since ab is negative, a and b have the opposite signs. Since a+b is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product -105.

$$1,-105 3,-35 5,-21 7,-15$$

Calculate the sum for each pair.

$$1-105=-104 3-35=-32 5-21=-16 7-15=-8$$

The solution is the pair that gives sum -8.

$$a=-15 b=7$$

Rewrite factored expression \left(x+a\right)\left(x+b\right) using the obtained values.

$$\left(x-15\right)\left(x+7\right)$$

To find equation solutions, solve x-15=0 and x+7=0.

$$x=15 x=-7$$

Steps Using Factoring By Grouping

Subtract 121 from both sides.

$$x^{2}-8x+16-121=0$$

Subtract 121 from 16 to get -105.

$$x^{2}-8x-105=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as x^{2}+ax+bx-105. To find a and b, set up a system to be solved.

$$a+b=-8 ab=1\left(-105\right)=-105$$

Since ab is negative, a and b have the opposite signs. Since a+b is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product -105.

$$1,-105 3,-35 5,-21 7,-15$$

Calculate the sum for each pair.

$$1-105=-104 3-35=-32 5-21=-16 7-15=-8$$

The solution is the pair that gives sum -8.

$$a=-15 b=7$$

Rewrite x^{2}-8x-105 as \left(x^{2}-15x\right)+\left(7x-105\right).

$$\left(x^{2}-15x\right)+\left(7x-105\right)$$

Factor out x in the first and 7 in the second group.

$$x\left(x-15\right)+7\left(x-15\right)$$

Factor out common term x-15 by using distributive property.

$$\left(x-15\right)\left(x+7\right)$$

To find equation solutions, solve x-15=0 and x+7=0.

$$x=15 x=-7$$

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.

$$x^{2}-8x+16=121$$

Subtract 121 from both sides of the equation.

$$x^{2}-8x+16-121=121-121$$

Subtracting 121 from itself leaves 0.

$$x^{2}-8x+16-121=0$$

Subtract 121 from 16.

$$x^{2}-8x-105=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -8 for b, and -105 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-8\right)±\sqrt{\left(-8\right)^{2}-4\left(-105\right)}}{2}$$

Square -8.

$$x=\frac{-\left(-8\right)±\sqrt{64-4\left(-105\right)}}{2}$$

Multiply -4 times -105.

$$x=\frac{-\left(-8\right)±\sqrt{64+420}}{2}$$

Add 64 to 420.

$$x=\frac{-\left(-8\right)±\sqrt{484}}{2}$$

Take the square root of 484.

$$x=\frac{-\left(-8\right)±22}{2}$$

The opposite of -8 is 8.

$$x=\frac{8±22}{2}$$

Now solve the equation x=\frac{8±22}{2} when ± is plus. Add 8 to 22.

$$x=\frac{30}{2}$$

Divide 30 by 2.

$$x=15$$

Now solve the equation x=\frac{8±22}{2} when ± is minus. Subtract 22 from 8.

$$x=-\frac{14}{2}$$

Divide -14 by 2.

$$x=-7$$

The equation is now solved.

$$x=15 x=-7$$

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.

$$x^{2}-8x+16=121$$

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}.

$$\left(x-4\right)^{2}=121$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-4\right)^{2}}=\sqrt{121}$$

Simplify.

$$x-4=11 x-4=-11$$

Add 4 to both sides of the equation.

$$x=15 x=-7$$