Solve x^2+2x+1=15

$$x ^ { 2 } + 2 x + 1 = 15$$

Solve for x (complex solution)

$$x=\sqrt{15}-1\approx 2.872983346$$

$$x=-\left(\sqrt{15}+1\right)\approx -4.872983346$$

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.

$$x^{2}+2x+1=15$$

Subtract 15 from both sides of the equation.

$$x^{2}+2x+1-15=15-15$$

Subtracting 15 from itself leaves 0.

$$x^{2}+2x+1-15=0$$

Subtract 15 from 1.

$$x^{2}+2x-14=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, 2 for b, and -14 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-2±\sqrt{2^{2}-4\left(-14\right)}}{2}$$

Square 2.

$$x=\frac{-2±\sqrt{4-4\left(-14\right)}}{2}$$

Multiply -4 times -14.

$$x=\frac{-2±\sqrt{4+56}}{2}$$

Add 4 to 56.

$$x=\frac{-2±\sqrt{60}}{2}$$

Take the square root of 60.

$$x=\frac{-2±2\sqrt{15}}{2}$$

Now solve the equation x=\frac{-2±2\sqrt{15}}{2} when ± is plus. Add -2 to 2\sqrt{15}.

$$x=\frac{2\sqrt{15}-2}{2}$$

Divide -2+2\sqrt{15} by 2.

$$x=\sqrt{15}-1$$

Now solve the equation x=\frac{-2±2\sqrt{15}}{2} when ± is minus. Subtract 2\sqrt{15} from -2.

$$x=\frac{-2\sqrt{15}-2}{2}$$

Divide -2-2\sqrt{15} by 2.

$$x=-\sqrt{15}-1$$

The equation is now solved.

$$x=\sqrt{15}-1 x=-\sqrt{15}-1$$

Steps for Completing the Square

Factor x^{2}+2x+1. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+1\right)^{2}=15$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+1\right)^{2}}=\sqrt{15}$$

Simplify.

$$x+1=\sqrt{15} x+1=-\sqrt{15}$$

Subtract 1 from both sides of the equation.

$$x=\sqrt{15}-1 x=-\sqrt{15}-1$$

Solve for x

$$x=\sqrt{15}-1\approx 2.872983346$$

$$x=-\sqrt{15}-1\approx -4.872983346$$

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.

$$x^{2}+2x+1=15$$

Subtract 15 from both sides of the equation.

$$x^{2}+2x+1-15=15-15$$

Subtracting 15 from itself leaves 0.

$$x^{2}+2x+1-15=0$$

Subtract 15 from 1.

$$x^{2}+2x-14=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, 2 for b, and -14 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-2±\sqrt{2^{2}-4\left(-14\right)}}{2}$$

Square 2.

$$x=\frac{-2±\sqrt{4-4\left(-14\right)}}{2}$$

Multiply -4 times -14.

$$x=\frac{-2±\sqrt{4+56}}{2}$$

Add 4 to 56.

$$x=\frac{-2±\sqrt{60}}{2}$$

Take the square root of 60.

$$x=\frac{-2±2\sqrt{15}}{2}$$

Now solve the equation x=\frac{-2±2\sqrt{15}}{2} when ± is plus. Add -2 to 2\sqrt{15}.

$$x=\frac{2\sqrt{15}-2}{2}$$

Divide -2+2\sqrt{15} by 2.

$$x=\sqrt{15}-1$$

Now solve the equation x=\frac{-2±2\sqrt{15}}{2} when ± is minus. Subtract 2\sqrt{15} from -2.

$$x=\frac{-2\sqrt{15}-2}{2}$$

Divide -2-2\sqrt{15} by 2.

$$x=-\sqrt{15}-1$$

The equation is now solved.

$$x=\sqrt{15}-1 x=-\sqrt{15}-1$$

Steps for Completing the Square

Factor x^{2}+2x+1. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+1\right)^{2}=15$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+1\right)^{2}}=\sqrt{15}$$

Simplify.

$$x+1=\sqrt{15} x+1=-\sqrt{15}$$

Subtract 1 from both sides of the equation.

$$x=\sqrt{15}-1 x=-\sqrt{15}-1$$