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# Chapter 21: Functions and relationships (Part 2)

## 21.1 Introduction

When working with functions that we apply to real-life situations, we will often need to work with formulas that describe the relationship between multiple variables. For example, in physics, Newton’s formula that describes second law of motion looks like this:

$F = ma$

In this formula, there are three quantities: force ($$F$$), mass ($$m$$) and acceleration ($$a$$). If we know mass and acceleration, we can find the force. If we know the force and the mass of the moving object, then we can also find its acceleration:

$a = \frac{F}{m}$

In Chapter 5, we looked at basic functions and relationships. In this chapter, the focus is on using formulas to find output values from given input values, as well as equivalent forms of descriptions of the same relationship.