Slope of a Line
Definition of the Slope of a Line
The slope of a straight line is a measure of how steep a line is and in general it is a measure of change. It is an extremely important concept in mathematics.
The three lines shown below have different properties: line \( L_1 \) is steeper than line \( L_2 \). Lines \( L_1 \)and \( L_2 \) rise but line \( L_3 \) falls. A parameter is needed to measure all these properties of a line quantitatively.
Let us consider the two points \( P_1(x_1,y_1) \) and \( P_2(x_2,y_2) \) on the line shown below. As we move from point \( P_1 \) to point \( P_2 \); to a change \( \Delta y = y_2  y_1\) in the vertical direction corresponds a change \( \Delta x = x_2  x_1\) in the horizontal direction.
The slope \( m \) of a line is defined as the ratio of the change along the y axis and the change of the x axis
\[ m = \dfrac{\Delta y}{\Delta x} = \dfrac {y_2  y_1}{x_2  x_1} \]
Properties of the Slope of a Line
1) The slope of a line is constant and any two points on the line may be used to find its value.2) A line with positive slope rises from left to right.
3) A line with negative slope falls from left to right.
4) The slope of a horizontal line is equal to 0.
5) The slope of a vertical line in undefined,
6) Since the slope is a ratio, it is also called rate of change.
7) Two lines with slopes \( m_1 \) and \(m_2 \) are parallel (they do not have a point of intersection) if and only if \( m_1 = m_2 \)
8) Two lines with slopes \( m_1 \) and \(m_2 \) are perpendicular (they have a point of intersection and make a right angle) if and only if \( m_1 \times m_2 =  1 \)
9) Although gradient and slope are sometimes used interchangeably, the gradient is a vector and the slope is a scalar. The magnitude of the gradient of a line is equal to the absolute value of the slope of this line.
Example 1 Find slopes
Find the slope of the lines \( L_1, L_2, L_3, L_4 \) through the given pair of points and describe the lines as rising, falling, horizontal or vertical and plot the given pair of points and the corresponding line in the same system of coordinates.
 \( L_1: (2 , 3) , (4 , 9) \)
 \( L_2: (1 , 0) , (6 , 8) \)
 \( L_3: (5 , 1) , (5 , 3) \)
 \( L_4: (7 , 3) , (5 , 3) \)
Solution to Example 1
 \( L_1: m_1 = \dfrac{9  3}{4  2} = 3\) , the line has a positive slope and therefore rises.
 \( L_2: m_2 = \dfrac{8  0}{6  (1)} = \dfrac{8}{7} \) , the line has a negative slope and therefore falls.
 \( L_3: m_3 = \dfrac{3  1}{ 5  (5) } = \dfrac{2}{0} = \text{undefined} \) , the slope is undefined and therefore the line is vertical.

\( L_4: m_3 = \dfrac{3  (3)}{5  7 } = \dfrac{0}{12} = 0 \) , the slope is equal to 0 and therefore the line is horizontal.
The graphs of the four lines and the pairs of points are shown below where all the above answers are confirmed.
Example 2 slopes of parallel and perpendicular lines
Find the slope of the lines \( L_1, L_2, L_3\) through the given pair of points and describe the lines as parallel or perpendicular. Graph the three lines in the same system of coordinates.
 \( L_1: (0 , 0) , (2 , 4) \)
 \( L_2: (0 , 0) , (  2 , 1) \)
 \( L_3: (1 , 1) , (3 , 5) \)
Solution to Example 2
 \( L_1: m_1 = \dfrac{4  0}{2  0} = 2 \)
 \( L_2: m_2 = \dfrac{1  0}{2  0} = \dfrac{1}{2} \)

\( L_3: m_3 = \dfrac{5  1}{3  1} = 2 \)
Since \(m_1 = m_3 \) , lines \( L_1 \) and \( L_3 \) are parallel.
Since \(m_1 \times m_2 = 2 \times (1/2) =  1 \) and \(m_3 \times m_2 = 2 \times (1/2) =  1 \), \( L_1 \) and \( L_2 \) are perpendicular and \( L_3 \) and \( L_2 \) are also perpendicular.
The graphs of the three lines are shown below.
Example 3
Two lines \( L_1 \) and \( L_2 \) are defined by the pair of points: \( L_1: (1,1), (2,3) \) and \( L_2: (1,2) , (4,7) \)
a) Find the slope of the two lines. Which of the two lines rises faster?
b) Graph the two lines and confirm graphically your answer to part a).
Solution to Example 3
a) \( L_1: m_1 = \dfrac{3  (1)}{2 (1)} = \dfrac{4}{3} \)
\( L_2: m_2 = \dfrac{7  2}{4  1} = \dfrac{5}{3} \)
Both lines have positive slopes and therefore rise. But because the slope \( m_2 = \dfrac{5}{3} \) is larger than the slope \( m_1 = \dfrac{4}{3} \), line \( L_2 \) rises faster than line \( L_1 \).
b) The graphs of the two lines are shown below and the answer to part a) is confirmed.
Interactive Tutorial to Explore the Slope of a LineAn app that can be used to explore the slope of a line interactively using the mousse is shown below. Follow the instructions below and explore the slope.Given two points \( A(x_A , y_A) \) and \( B(x_B , y_B) \), the slope m of the line through the two points is given by the formula (see formula above): \( m = \dfrac{y_B  y_A}{x_B  x_A} \)
More References and LinksSolve Slope ProblemsSlopes of Parallel Lines Questions Easy to use calculator to find slope and equation of a line through two points.Two Points Calculator 