Formulas
Part of the Expressions & Data section of Coddy's Pre-Algebra journey. Lesson 9 of 52.
A formula is an expression with a job: it says how to work out a quantity from measurements. The perimeter of a rectangle, the distance all the way around it, is P = 2L + 2W. L is the length and W the width, and capital letters named after what they measure keep the formula easy to read.
A rectangle 6 metres long and 5 metres wide has L = 6 and W = 5. Substitute both: P = 2 × 6 + 2 × 5. Multiply first, so the line becomes P = 12 + 10, and the perimeter is P = 22 metres. The answer is in the same unit as the measurements.
Why does the formula have 2L and 2W? A rectangle has two sides of each length, and the perimeter walks along all four. One formula works for every rectangle; only the numbers you substitute change.
Challenge
A rectangular garden is 9 metres long and 4 metres wide. Use the formula P = 2L + 2W with L = 9 and W = 4 to find its perimeter P in metres. Write P equal to the formula with the numbers in, then work it out.
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Expressions & Data
1Variables and Expressions
What a Variable IsWriting ExpressionsExpressions from StoriesTwo OperationsRecap: Expressions2Evaluating Expressions
Substituting a ValueNegative InputsPowers in ExpressionsFormulasRecap: Evaluating5Patterns and Sequences
Continuing a PatternThe Common DifferenceThe nth TermWriting the RuleRecap: Sequences3The Distributive Property
Number PropertiesMental MathDistributing LettersDistributing a MinusRecap: Distributing