Home » Math Vocabulary » Slope Intercept Form – Definition, Formula, Facts, Examples

- What Is the Slope-Intercept Form of a Straight Line?
- Slope-Intercept Form: Formula
- Converting Standard Form to slope-intercept Form
- Solved Examples of Slope-Intercept Form of Line
- Practice Problems on slope-intercept Form of a Line
- Frequently Asked Questions on Slope-intercept Form of a Line

## What Is the Slope-Intercept Form of a Straight Line?

**The slope-intercept form of the equation of a straight line is used to write the equation of a **line** using its slope and the y-intercept. It is usually given by **y = mx + b**.**

The slope of a line is given by the rise-over-run ratio. The y-intercept is the point where the line intersects with the Y-axis.

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## Slope-Intercept Form: Definition

The slope intercept form is a way of writing the equation of the straight line using the slope and the y-intercept of the line.

The equation of a line with slope m and y-intercept b is written in the slope-intercept form as

$y = mx + b$

where

(x,y) represents the coordinates of any point on the line.

However, the slope-intercept formula of the straight line cannot be used to write the equation of a vertical line because the slope of the vertical line is not defined.

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## Slope-Intercept Form: Formula

The equation of a straight line in the slope-intercept form is given by

$y = mx + b$

Where

**m** equals the slope

**b **represents the y-intercept of the straight line

**(x,y)** determines each point on the straight line and is considered as variable.

## Derivation of Formula For Slope-Intercept Form

Let us assume that a line with a slope m has the y-intercept b. It means that the line intersects the y-axis at the point (0, b).

Let (x,y) be another random point on the line.

Thus, we have the coordinates of two points on the line.

$(x_{1},\; y_{1}) = (0,\;b)$ and $(x_{2},\; y_{2}) = (x,\;y)$

*Alt Text: Graphical representation of the slope-intercept form equation of a straight line.*

We know that the slope of a line passing through the points $(x_{1},\; y_{1})$ and $(x_{2},\; y_{2})$ is given by

$m = \frac{(y_{2}\;-\;y_{1})}{(x_{2}\;-\;x_{1})}$

For $(x_{1},\; y_{1}) = (0,\;b)$ and $(x_{2},\; y_{2}) = (x,\;y)$, we can write

$m = \frac{(y \;-\; b)}{(x \;-\; 0)}$

$m = \frac{(y \;-\; b)}{x}$

$mx = (y\;-\;b)$

$y = mx + b$

This is called the slope-intercept form of the equation of a straight line.

## Slope-Intercept Form: Examples

- Suppose the slope is $-2$ and the y-intercept is 5. The equation of the line is given by

$y = \;-\;2x + 5$

- Slope $= 5$ and the line passes through the origin (0, 0). The equation of line is given by

$y = 5x$

## Straight Line Equation Using Slope-Intercept Form

To evaluate the line equation with an arbitrary inclination, two quantities are required, i.e.,

- Inclination/slope of the line
- Arrangement of the line based on the coordinates of each point on the y-axis

Lines can be formed using these two key parameters. Let us know the steps to evaluate **what the slope-intercept form of a line is**.

**Step 1:** Find the slope (m) of the line using the given information.

- If θ is the angle the line makes with the positive x-axis, the slope of the straight line = tan θ.
- If $(x_{1},\; y_{1})$ and $(x_{2},\;y_{2})$ are two points on the line, then the slope of the straight line $= \frac{(y_{2}\;-\;y_{1})}{(x_{2}\;-\;x_{1})}$.

**Step 2:** Note down the y-intercept (b).

**Step 3:** Substitute the values in the slope-intercept form $y = mx + b$ to find the equation of the straight line.

## Converting Standard Form to slope-intercept Form

A standard form equation of a line can easily be converted to the **slope-intercept form** by comparison and rearrangement of the points. Let us explore the standard equation represented as follows:

$Ax + By + C = 0$

where A, B, C are constants;

A, B cannot be simultaneously 0.

Let’s rewrite it as

$By = \;-\; Ax\;-\;C$

B rearranged from LHS to RHS, i.e., from multiplication to division on the other side, we get

$y = (\frac{-\;A}{B})x + (\frac{-\;C}{B})$

Therefore, we get

- slope $= m = (\frac{-\;A}{B})$
- y-intercept $= b = (\frac{-\;C}{B})$

## Facts about the Slope-Intercept Form

- The slope-intercept form is also written as $y = mx + c$, where m is the slope and c is the y-intercept.
- The slope-intercept form of the equation of a line having slope m and passing through the origin is$y = mx$.

## Conclusion

In this article, we learned about the slope-intercept form, which is used to find the equation of the straight line using the slope and the y-intercept. It is given by y = mx + b. We learned the formula, and its derivation. Let’s solve a few examples and practice problems to master these concepts.

## Solved Examples of Slope-Intercept Form of Line

**1. Evaluate the straight line equation where slope **$m = 4$** passes via the point **$(\;-\;1, \;-\;3)$**.**

**Solution:**

Let the equation of line be $y = mx + c$.

Slope of the line is $m = 4$

The line passes through the point $(\;-\;1,\; -\;3)$. Let’s use it to find the y-intercept.

Substitute $y = \;-\;3$ and $x = \;-\;1$ in $y = mx + c$.

Putting the values in the above slope-intercept formula, we will obtain

$\;-\;3 = 4(\;-\;1) + b$

$\;-\;3 = \;-\;4 + b$

$b = \;-\;3 + 4$

$b = 1$

Thus, we have $m = 4$ and $b = 1$.

Using the slope-intercept form, we write the equation of the line as

$y = 4x + 1$

**2. Evaluate the equation of the straight line when **$m = \;-\;2$** and passes through the point **$(3,\; -\;4)$**.**

**Solution:**

Let the equation of line be $y = mx + b$.

The line passes through $(3,\; -\;4)$**.**

Thus, the point satisfies the equation.

$-\;4 = \;-\;2 (3) + b$

$\;-\;4 = \;-\;6 + b$

$b = \;-\;4 + 6$

$b = 2$

Thus, the y-intercept is 2.

Therefore, the required slope-intercept form equation of the straight line will be written as $y = \;-\;2x + 2$

**3.** **Write the equation of line **$7x + 8y \;-\; 1 = 0$** in the slope-intercept form. Find the slope and y-intercept.**

**Solution:**

We want to write the equation of the line in the form $y = mx + b$.

$7x + 8y \;-\; 1 = 0$

$\Rightarrow 8y = \;-\;7x + 1$

$\Rightarrow y = \frac{-\;7x}{8} + \frac{1}{8}$

The slope of the line is $\frac{-\;7x}{8}$.

The y-intercept is $\frac{1}{8}$.

## Practice Problems on slope-intercept Form of a Line

1

### What is the slope-intercept form of a line formula?

$y = m + xb$

$y = x + mb$

$y = mx + b$

$y = mx \times b$

CorrectIncorrect

Correct answer is: $y = x + mb$

The slope-intercept form of a line is given by $y = mx + b$.

2

### If slope $= 1$ and y-intercept $= 1$, the equation of line is

$y = 1$

$x + y = 1$

$y = x + 1$

$y = x \;-\; 1$

CorrectIncorrect

Correct answer is: $y = x + 1$

Substitute $m = 1$ and $b = 1$ in $y = mx + b$, we get $y = x + 1$.

3

### The equation of a line with slope m and passing through the origin is

$y = m + x$

$y = mx$

$y = m$

$my = x$

CorrectIncorrect

Correct answer is: $y = mx$

The slope of a line with slope m that passes through the origin is $y = mx$.

4

### What will be the y-intercept of equation $2x + 5y \;-\; 1 = 0$?

$\frac{1}{5}$

$\frac{2}{5}$

$-\;\frac{2}{5}$

$-\;\frac{1}{5}$

CorrectIncorrect

Correct answer is: $\frac{1}{5}$

$5y = 1\;-\; 2x$

$y = \frac{\;-\;2x}{5} + \frac{1}{5}$

Thus, y-intercept $= \frac{1}{5}$

## Frequently Asked Questions on Slope-intercept Form of a Line

**When can we apply the slope-intercept form?**

We use the slope-intercept form to find the equation of the line when the slope and the y-intercept is known or can be calculated using the available information.

**What is the purpose of the slope in the slope-intercept form?**

The slope of the line determines if the line is increasing or decreasing and how steep it is established. It is denoted by m and represents how quickly y-axis coordinates change with the slight change in x-axis coordinates.

**How is the point-slope form different from the slope-intercept form?**

Slope-intercept form and point-slope form are two different forms to write the equation of a straight line.

Point-slope form: $y \;-\; y_{1} = m(x \;-\; x_{1})$, where m the slope and $(x_{1},\;y_{1})$ are the coordinates of any arbitrary point on the line.

slope-intercept form: $y = mx + b$, where m the slope and b is the y-intercept.

**Why is slope-intercept form not used for vertical lines?**

The slope-intercept form is not used for vertical lines, as the slope of a vertical line is not defined.

**What is the main advantage of the slope-intercept form?**

We can easily identify the slope and the y-intercept of the line by looking at the slope-intercept form.