Area by Subtracting

Part of the Polygons & Quadrilaterals section of Coddy's Geometry journey. Lesson 49 of 61.

Sometimes it is easier to start big and take away. A rectangle with a corner cut off is the whole rectangle minus the corner triangle. Area of the parts works both ways: if the whole and one piece are known, the other piece is the difference.

$A 10 by 8 rectangle with a right triangle cut from the corner C; the triangle has legs 4 and 6$

Here the rectangle is 10 by 8, area 80. The corner triangle ECF has a right angle at C and legs 4 and 6, so its area is 1/2 × 4 × 6 = 12. The piece that is left has area 80 - 12 = 68. In a right triangle the two legs are a base and its height.

Challenge

Rectangle ABCD has AB = 9 and BC = 6. The corner triangle ECF, with EC = 4 and CF = 3, is cut off along EF. Find the area of the five-sided piece ABFED that is left, then press Check.

Try it yourself

$Find the marked part of the figure$

Find the marked part of the figure

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Polygons & Quadrilaterals

2Regular Polygons

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5Rectangles, Rhombi, Squares

Rectangles The Diagonal of a Rectangle Rhombus Angles Squares Recap: Special Parallelograms

8Area of Parallelograms

Area of a Rectangle A Side from the Area Slide the Top Area of a Parallelogram Recap: Parallelogram Area

11Polygon Chases

Parallel Sides in a Pentagon Marks, Not Looks Pentagon with x Regular Shapes Together Recap: Polygon Chases