# 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:

• Figuring the interest by using the simple interest or compound interest formula
• Determining the height of an object after a certain amount of time
• Computing how far you've traveled given rate and time
• Calculating the sum of the measures of the angles in a polygon
• Finding the average or weighted average of items
• Summing a series of numbers
• Figuring out the value of a term in a sequence of numbers

## What to Watch Out For

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

• Assigning the correct value to the different variables in a formula
• Changing units, if necessary, to have consistency in the formula's input values
• Performing the operations correctly by using the order of operations

## 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% ...

Get Algebra I: 1,001 Practice Problems For Dummies now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.