Differences Between Quadratic & Linear Equations

Differences Between Quadratic & Linear Equations
••• tcantaffa/iStock/GettyImages

A linear equation in two variables doesn't involve any power higher than one for either variable. It has the general form:

Ax + By + C = 0

where A, ​B​ and ​C​ are constants. It's possible to simplify this to

y = mx + b\text{ where } m = \frac{ −A}{B}

and ​b​ is the value of ​y​ when ​x​ = 0. A quadratic equation, on the other hand, involves one of the variables raised to the second power. It has the general form

y = ax^2 + bx + c

Apart from the adding complexity of solving a quadratic equation compared to a linear one, the two equations produce different types of graphs.

TL;DR (Too Long; Didn't Read)

Linear functions are one-to-one while quadratic functions are not. A linear function produces a straight line while a quadratic function produces a parabola. Graphing a linear function is straightforward while graphing a quadratic function is a more complicated, multi-step process.

Characteristics of Linear and Quadratic Equations

A linear equation produces a straight line when you graph it. Each value of ​x​ produces one and only one value of ​y​, so the relationship between them is said to be one-to-one. When you graph a quadratic equation, you produce a parabola that begins at a single point, called the vertex, and extends upward or downward in the ​y​ direction. The relationship between ​x​ and ​y​ is not one-to-one because for any given value of ​y​ except the ​y​-value of the vertex point, there are two values for ​x​.

Solving and Graphing Linear Equations 

Linear equations in standard form (​Ax​ + ​By​ + ​C​ = 0) are easy to convert to convert to slope intercept form (​y​ = ​mx​ +​b​), and in this form, you can immediately identify the slope of the line, which is ​m​, and the point at which the line crosses the ​y​-axis. You can graph the equation easily, because all you need are two points. For example, suppose you have the linear equation

y = 12x + 5

Choose two values for ​x​, say 1 and 4, and you immediately get the values 17 and 53 for ​y​. Plot the two points (1, 17) and (4, 53), draw a line through them, and you're done.

Solving and Graphing Quadratic Equations

You can't solve and graph a quadratic equation quite as simply. You can identify a few general characteristics of the parabola by looking at the equation. For example, the sign in front of the ​x2 term tells you whether the parabola opens up (positive) or down (negative). Moreover, the coefficient of the ​x2 term tells you how wide or narrow the parabola is -- large coefficients denote wider parabolas.

You can find the the ​x​-intercepts of the parabola by solving the equation for ​y​ = 0 :

ax^2 + bx + c = 0

and using the quadratic formula

x = \frac{ −b ± \sqrt{b^2 − 4ac}}{2a}

You can find the vertex of a quadratic equation in the form

y = ax^2 + bx + c

by using a formula derived by completing the square to convert the equation into a different form. This formula is


It gives you the ​x​-value of the intercept, which you can plug into the equation to find the ​y​-value.

Knowing the vertex, the direction in which the parabola opens and the ​x​-intercept points gives you enough of an idea of the appearance of the parabola to draw it.

Related Articles

How to Convert an Equation Into Vertex Form
How to Find the Vertex of a Parabola Equation
How to Figure Out the Slope of a Line
How to Solve Binomial Equations by Factoring
How to Find Equations of Tangent Lines
How to Solve a Parabola
How to Write a Prediction Equation for a Scatter Plot
How to Find Slope From an Equation
How to Find a Parallel Line
What Is the Square Root Method?
How to Find X-Intercept & Y-Intercept
How to Use Elimination to Solve the Linear Equation
How to Calculate Regression Coefficient
How to Find an Ordered Pair From an Equation
How to Calculate Half of a Parabolic Curve
How to Find the Line of Symmetry in a Quadratic Equation
How to Interpret Linear Equations
Difference Between Parabola and Line Equation
How to Determine the Y-Intercept of a Trend Line
How to Graph and Find the Solution on a Calculator