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Quadratic Equations

Completing the Square

Quadratic expressions, functions, and equations can all be manipulated in useful ways with a method called completing the square. This method is based on the concept of perfect square trinomials.


Perfect Square Trinomial

Just as a perfect square integer, such as can be written as the square of integer square roots, a perfect square trinomial is a trinomial which can be written as the square of a binomial.

There are two general types of perfect square trinomials that can be useful when dealing with quadratic functions or quadratic equations. This process involves factoring the quadratic trinomial and rewriting the perfect square trinomial as the square of a binomial.


Factoring a Perfect Square Trinomial

To be able to rewrite an expression to include a perfect square trinomial, it is first necessary to be able to recognize them. This can be achieved by expanding the square of a general binomial. The factoring is then done in the opposite direction. There are two kinds of perfect square trinomials, leading to different signs between the terms in the binomial.


If a trinomial is in the form where and are variables or positive numbers, it is a perfect square trinomial that can be factored as This is shown by expanding the squared binomial.

Thus, the trinomial and the square are equal.


If a trinomial instead is in the form it is also a perfect square trinomial and can be factored as This is shown by expanding the squared binomial.

The trinomial and the square are indeed equal.


The expressions in the table are perfect square trinomials. Complete the expressions to ensure equivalence between corresponding standard forms and factored forms.

standard form factored form
__ ___
___ __ ___
___ __ ___
Show Solution

Here the same expressions are written both in standard form and in factored form, but one or more terms are missing in each expression. To identify the missing term(s) the rules for factoring a perfect square trinomial are useful.


To help us identify the terms in the rule in the first example, we can for the expression rewrite as and as

__ ___

To make the expressions match each other we will fill in where the blanks are.

We can use this reasoning to complete the other perfect square trinomials in the table. Next, we'll consider the second row.


To fill in the second blank we use that must be which gives us that When we study the first blank we can match with What is left is

This we prefer writing as Let's continue with the third row

_____ ___

Here the terms and match each other. Since the terms and must match each other we find that or Let's use this to fill in the blanks in the expression.

By squaring we get that the second row reads We are now going to deal with the last row in the table.

_____ ___

The lines match each other when giving us the first blank and the second blank Let's write them together with the blanks filled in.

We'll summarize our results by adding the found values to the table.

standard form factored form


Completing the Square

Completing the square is a method by which a quadratic expression is rewritten as a difference of a perfect square trinomial and a constant. Commonly this is done by adding and subtracting a constant, By choosing the constant, as the quadratic expression becomes a difference of a perfect square trinomial and a constant. The quadratic expression can then be factored and written and written in vertex form.


Completing the Square, In-Depth


Quadratic Expressions in the Form

When completing the square for a quadratic expression in the form the constant must also be taken into account. When adding and subtracting the term the quadratic expression gets a constant term of



After completing its square, a quadratic expression can be rewritten into vertex form by factoring the perfect square trinomial.


Solving Quadratic Equations

Completing the square can be used when solving quadratic equations. An equation in the form is then rewritten by isolating the and terms. By adding the term to both sides of the equation, a perfect square trinomial is formed. The equation can then be rewritten by factoring the perfect square trinomial. When the equation is written on this form it can be solved with square roots.


Geometric Interpretation of Completing the Square

It can be helpful to visualize what's going on when completing the square. Press the button below to see an animation of this concept.
Complete the square


Completing the Square

This is a method by which a quadratic expression is rewritten as the difference of the square of a binomial and a constant. To complete the square, there are five steps to follow.


Factor Out the Coefficient of

It is easier to complete the square when the expression is written in the form Therefore, if the coefficient of is not it should be factored out. For the simplicity of the following steps, it will be assumed that For values of other than the same steps should be performed. The only difference is that the new coefficients — instead of and instead of — will be used.


Identify the Constant Needed to Complete the Square

The constant needed to complete the square can now be identified by focusing on the term, while ignoring the rest. One way to find this constant is by squaring half the coefficient of the term, which in this case is Note that leaving the constant as a power makes the next steps easier to perform.


Complete the Square

The square can now be completed by adding and subtracting the constant found in Step Note that the value of the original expression will not be changed since the result of adding and subtracting the same value is equal to The first three terms form a perfect square trinomial, which can be factored as the square of a binomial. The other two terms do not contain the variable so their value is constant.


Factor the Perfect Square Trinomial
The perfect square trinomial can now be factored and rewritten as the square of a binomial.
The process of completing the square is now finished.


Simplify the expression
Finally, if needed, the expression can be simplified. In case was not equal to now is a good time to remove the parentheses and multiply the obtained expression by
Derive function


The method of completing the square is often used to solve quadratic equations. To do so, the resulting square of a binomial should be written on one side of the equation, while the constant should be on the opposite side. Then, the solutions are found by taking the square root of each side of the equation.


Instead of a purely algebraic approach, it can be helpful to visualize the process of completing the square geometrically.
The animation shows that the process of completing the square can be understood as finding the dimensions of the that completes an incomplete square.

Consider the quadratic function Complete the square to determine the vertex and zeros of the parabola. Then, use them to draw the graph.

Show Solution
To begin, we'll complete the square on by focusing on the coefficient of which is Since is the constant term that will create a perfect square trinomial.
Notice that is now written in vertex form. It can be seen that 's vertex is To find the zeros we can set equal to and solve.
Thus, the zeros of the parabola are and Lastly, we can use the vertex and the zeros to draw the graph of
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