Question

How do you complete the square to solve \[{x^2} + 4x = 12\]?

Answer 1

Hint: We first make the coefficient of \[{x^2}\] as 1 by dividing the complete equation by the coefficient. Then shift the constant value to the right hand side of the equation. Add the square of half value of coefficient of x on both sides of the equation. Use the identity \[{(a + b)^2} = {a^2} + {b^2} + 2ab\] to write the left side of the equation. Take the square root on both sides of the equation and write the value by solving the equation. Take LCM on the left side and cross multiply values.

Complete step-by-step answer:
We are given the quadratic equation \[{x^2} + 4x = 12\] … (1)
We can write quadratic equation as \[{x^2} + 4x - 12 = 0\]
We compare the equation with general quadratic equation i.e. \[a{x^2} + bx + c = 0\]
Coefficient of \[{x^2}\] is 1, coefficient of x is 4 and constant value is -12.
Since we see in equation (1) coefficient of \[{x^2}\] is 1 and the constant term is on the right hand side, we will proceed with next steps.
The coefficient of x is 4, we divide the number by 2 and square it i.e. \[{\left( {\dfrac{4}{2}} \right)^2} = {2^2}\]
So, the value becomes 4
Add this term to both sides of the equation
\[ \Rightarrow {x^2} + 4x + 4 = 12 + 4\]
We can write left hand side of the equation using the identity \[{(a + b)^2} = {a^2} + {b^2} + 2ab\]and simplify right hand side by simply adding the terms
\[ \Rightarrow {\left( {x + 2} \right)^2} = 16\]
Take square root on both sides of the equation
\[ \Rightarrow \sqrt {{{\left( {x + 2} \right)}^2}} = \sqrt {16} \]
We know \[16 = {4^2}\]
\[ \Rightarrow \sqrt {{{\left( {x + 2} \right)}^2}} = \sqrt {{4^2}} \]
Cancel square root by square power on both sides of the equation
\[ \Rightarrow \left( {x + 2} \right) = \pm 4\]
Now equate left side with right side of the equation
\[ \Rightarrow x + 2 = 4\] and \[x + 2 = - 4\]
Shift constant values to right side
\[ \Rightarrow x = 4 - 2\] and \[x = - 4 - 2\]
\[ \Rightarrow x = 2\] and \[x = - 6\]

\[\therefore \]The solution of \[{x^2} + 4x = 12\]is \[x = 2\] and \[x = - 6\]

Note:
Many students make mistakes by squaring the value of b and then divide by 2 instead they have to first divide by 2 and then square it. Also, keep in mind when shifting values from one side of the equation to another side of the equation, always change sign from positive to negative and vice-versa.