# Chapter 18

# Using Established Formulas

** A** formula is a rule that describes a situation that happens consistently or exists without variation. One of the first formulas that people learn is that for the area of a rectangle — just multiply the length by the width. The trick to using formulas is to understand what the different symbols represent and then to be able to apply the mathematical rules correctly.

## The Problems You'll Work On

The majority of the problems in this chapter involve simply determining which formula to use, where to use it, and applying the following techniques:

## What to Watch Out For

Whether you struggle remembering formulas or are a formula whiz, be sure you don't overlook the following:

## Getting Interested in Interest Problems

**766–769** *Solve each using the simple interest formula, I = Prt*.

** 766.** How much interest is earned if you invest $20,000 at 2.5% ...

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