Question

Solve the following quadratic equation by completing the square: x² + 12x = 15

Answer 1

Answer:

x = -1.14 or -13.14

Explanations:

The given equation is:

$x^2+12x\text{ = 15}$

Find the half of 12, and add the square to both sides of the equation.

That is add 6² to both sides

$x^2+12x\text{ + 6}^{2}\text{ = 15 + 6}^{2}$$\begin{gathered} (x+6)^2\text{ = 15 + 3}6 \\ (x+6)^2\text{ = 51} \\ \end{gathered}$

Find the square root of both sides:

$\begin{gathered} \sqrt[]{(x+6)^2_{}}=\pm\sqrt[]{51} \\ \text{x + 6 = }\pm\sqrt[]{51} \\ x\text{ = -6 }\pm\sqrt[]{51} \\ \text{x = }-6\pm7.14 \\ x\text{ = -6+7.14 = }1.14 \\ x\text{ = -6 - 7.14} \\ \text{x = -13.14} \end{gathered}$

x = -1.14 or -13.14