# How to Use the Difference Quotient to Find the Slope of a Line

The difference quotient is a mathematical expression that can help us find the slope of a line. The slope of a line is a measure of how steep and in what direction the line is going. In this article, we will learn what the difference quotient is, how to use it to find the slope of a line, and why it is useful in calculus.

### What is the Difference Quotient?

The difference quotient is defined as the ratio of the change in the output value of a function to the change in the input value of the function. In other words, it tells us how much the function changes when we change its argument by a small amount. The difference quotient can be written as:

�����(�+ℎ)−�(�)ℎfracf(x+h)−f(x)h

where �(�)f(x) is the function, �x is the input value, and ℎh is the small change in the input value.

The difference quotient can also be interpreted geometrically. If we have a function that represents a curve on a coordinate plane, then the difference quotient can be seen as the slope of the secant line that passes through two points on the curve: (�,�(�))(x,f(x)) and (�+ℎ,�(�+ℎ))(x+h,f(x+h)). The secant line is a straight line that cuts through the curve at two points. The slope of the secant line is the ratio of the vertical distance between the two points to the horizontal distance between them. This is exactly what the difference quotient calculates.

### How to Use the Difference Quotient to Find the Slope of a Line

If we have a function that represents a straight line on a coordinate plane, then we can use the difference quotient to find its slope. Since the line is straight, any two points on it will have the same slope. Therefore, we can choose any two points on the line and plug their coordinates into the difference quotient formula. For example, suppose we have a line with equation �=2�+5y=2x+5. We can choose any two points on this line, such as (1,7)(1,7) and (3,11)(3,11). Then we can use these points to find the slope of the line using the difference quotient:

�����(�+ℎ)−�(�)ℎ=�����(3)−�(1)3−1=����(2�����3+5)−(2�����1+5)2=����11−72=2fracf(x+h)−f(x)h=fracf(3)−f(1)3−1=frac(2times3+5)−(2times1+5)2=frac11−72=2

We can see that the slope of the line is 2, which matches with the coefficient of �x in the equation of the line. This method works for any linear function, as long as we choose two distinct points on it.

### Why is the Difference Quotient Useful in Calculus?

The difference quotient is not only useful for finding the slope of a line, but also for finding the derivative of any function. The derivative of a function is a measure of how fast and in what direction the function changes at any point. The derivative can be seen as the slope of the tangent line to the curve at that point. The tangent line is a straight line that touches the curve at one point and has the same direction as the curve at that point.

The difference quotient can help us find an approximation of the derivative by using a small value of ℎh. As we make ℎh smaller and smaller, we are moving closer and closer to finding the exact slope of the tangent line at �x. In fact, if we take the limit of the difference quotient as ℎh approaches zero, we get exactly the derivative of �(�)f(x) at �x. This can be written as:

�′(�)=���ℎ��0�����(�+ℎ)−�(�)ℎf′(x)=limhto0​fracf(x+h)−f(x)h

where �′(�)f′(x) is the notation for the derivative of �(�)f(x).

The derivative is an important concept in calculus because it tells us how a function behaves locally. It can help us find rates of change, optimize functions, model physical phenomena, and more.

### Summary

The difference quotient is a mathematical expression that can help us find:

• The slope of a line by using any two points on it
• The derivative of any function by taking its limit as ℎh approaches zero

The slope and the derivative are both measures of how steep and in what direction a function changes. They are useful for analyzing and understanding functions and their applications.