Mathematics
Grade 12
15 min
Solve a quadratic equation by completing the square
Solve a quadratic equation by completing the square
Tutorial Preview
1
Introduction & Learning Objectives
Learning Objectives
Transform a quadratic equation from standard form, `ax^2 + bx + c = 0`, into a perfect square form `(x + p)^2 = q`.
Calculate the constant term, `(b/2)^2`, needed to create a perfect square trinomial from an expression `x^2 + bx`.
Solve quadratic equations with rational, irrational, and complex roots using the completing the square method.
Solve equations where the leading coefficient `a` is not equal to 1 by first dividing the entire equation by `a`.
Derive the vertex form of a parabola, `y = a(x - h)^2 + k`, from the standard form `y = ax^2 + bx + c`.
Articulate the connection between the algebraic process of completing the square and the geometric properties of a parabola's vertex.
How can you algebraically force a quadratic expression into a perf...
2
Key Concepts & Vocabulary
TermDefinitionExample
Quadratic Equation (Standard Form)An equation that can be written in the form `ax^2 + bx + c = 0`, where `a`, `b`, and `c` are constants and `a ≠ 0`.`2x^2 - 5x + 3 = 0` is a quadratic equation where `a=2`, `b=-5`, and `c=3`.
Perfect Square TrinomialA trinomial that is the square of a binomial. It follows the pattern `A^2 + 2AB + B^2 = (A + B)^2` or `A^2 - 2AB + B^2 = (A - B)^2`.`x^2 + 10x + 25` is a perfect square trinomial because it can be factored into `(x + 5)^2`.
Completing the SquareThe algebraic process of adding a specific constant term to a quadratic expression of the form `x^2 + bx` to transform it into a perfect square trinomial.To complete the square for `x^2 + 6x`, we add `(6/2)^2 = 9` to get `x^2 + 6x + 9`, which equals `(x + 3)^2`.
Vertex FormA form of...
3
Core Formulas
The Term to Complete the Square
For an expression `x^2 + bx`, the constant to add is `(b/2)^2`.
This is the core of the method. After isolating the `x^2` and `x` terms, identify the coefficient `b`, divide it by 2, and square the result. This value must be added to both sides of the equation to maintain balance.
Factored Perfect Square Form
`x^2 + bx + (b/2)^2 = (x + b/2)^2`
Once you have added the correct term to create a perfect square trinomial, it will always factor in this predictable way. The term inside the parentheses is always `x` plus half of the original `b` coefficient.
The Square Root Property
If `(x + p)^2 = q`, then `x + p = ±√q`.
After factoring the perfect square, use this property to eliminate the exponent. This step introduces the `±` symbol, which...
5 more steps in this tutorial
Sign up free to access the complete tutorial with worked examples and practice.
Sign Up Free to ContinueSample Practice Questions
Challenging
By completing the square for `x` in the equation `x^2 + kx + c = 0`, what are the solutions for `x` expressed in terms of `k` and `c`?
A.x = -k ± √(k^2 - 4c)
B.x = (-k ± √(k^2 - c)) / 2
C.x = -k/2 ± √(k^2 - c)
D.x = (-k ± √(k^2 - 4c)) / 2
Challenging
The solutions to a quadratic equation are found to be `x = 5 ± √3`. If the method used was completing the square, which of the following was the original equation in the form `x^2 + bx + c = 0`?
A.x^2 + 10x + 22 = 0
B.x^2 - 10x + 22 = 0
C.x^2 - 10x + 28 = 0
D.x^2 - 5x + 3 = 0
Challenging
For the quadratic function `y = -3x^2 + 6x - 7`, the vertex form `y = a(x - h)^2 + k` is found by completing the square. What is the value of the sum `a + h + k`?
A.-6
B.-8
C.5
D.-4
Want to practice and check your answers?
Sign up to access all questions with instant feedback, explanations, and progress tracking.
Start Practicing Free