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All courses Math Foundations · MF.GM.3 50 of 60
Use area formulas for rectangles, triangles, parallelograms, trapezoids, and composite figures.

Multiply length by width

Problem
A rectangle has a length of \(9~\text{cm}\) and a width of \(4~\text{cm}\). What is its area?
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Hint

Start with the rectangle area formula: length × width. Use 9 cm and 4 cm.

Area of a rectangle is found by multiplying its length by its width, not by adding the sides.

Solution walkthrough

01

Use the rectangle area formula

\[A=lw\]

Area counts the square units covering a rectangle and equals length times width.

02

Source the dimensions

\[l=9~\text{cm};~w=4~\text{cm}\]

The rectangle is nine centimeters long and four centimeters wide.

03

Substitute and calculate

\[A=(9~\text{cm})(4~\text{cm})=36~\text{cm}^2\]

Nine rows of four square centimeters total thirty-six square centimeters.

04

Check units and reasonableness

\[\text{cm}\times~\text{cm}=\text{cm}^2;~9\times~4=36\]

Multiplying two length dimensions correctly produces square units.

05

Report the area

\[36~\text{cm}^2\]

The rectangle covers thirty-six square centimeters.

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Another way

  1. Imagine a 9-by-4 grid and count 4+4+4+4+4+4+4+4+4=36 squares.

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Common mistake

Do not add 9 and 4 or double them; those operations contribute to perimeter, not area.

Solution walkthrough video