Find a Rectangle Dimension from Its Area

Supplied materials

• Paper and a pencil; drawing and spoken explanations are welcome. An adult may read directions and options aloud.
• Supplied worked-case record: A rectangle has area 84 square centimeters and length 12 cm.
• All quantities and shape descriptions are supplied in the worked example and questions; no purchase or personal information is needed.

Learn the idea

Rectangle area equals length times width. When the area and one dimension are known, division finds the other dimension. Perimeter is then computed from both lengths. Area uses square units, while each dimension and the perimeter use ordinary length units.

Divide the supplied area by the known side to find the missing side. Multiply both dimensions to check area. If asked for perimeter, add all four sides using the recovered dimension.

Use multiplication to verify the recovered dimension. Area and side lengths have different units. Write area = length × width and label each variable’s unit.

1. Write area = length × width and label each variable’s unit.
2. Follow the worked example. Point to each number or shape feature and say what it tells you.
3. Divide the supplied area by the known side to find the missing side.
4. Try the question and then the three practice questions. Check your answers and explain any change you make.

Check your reasoning

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Take it further

Area 90 square cm, width 9 cm: length 10 cm and perimeter 38 cm.

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