Half the Rectangle
Part of the Triangles section of Coddy's Geometry journey. Lesson 17 of 63.
The area of a shape is how much flat space it covers, counted in squares: square centimetres, square metres. A rectangle 8 by 5 covers 8 × 5 = 40 squares.
A triangle is half a rectangle. Draw the rectangle around a triangle, as wide as its base and as tall as its height. When the height lands on the base, it cuts the triangle into two right triangles, and each one is exactly half of its part of the rectangle. So the triangle covers half the rectangle:
A = 1/2 × b × h
The height is the dashed segment from the top corner that meets the base at a right angle. Here the base is 8 and the height is 5, so A = 1/2 × 8 × 5, which is 20 square units. On the stage, tap inside the triangle and pick Area of a triangle.
Challenge
△ABC has its height CD drawn. Find the area of △ABC: tap inside it, pick the rule, then press Check.
Try it yourself
Find the marked part of the figure
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Triangles
1Sides and Names
Three Sides, Three CornersSorting by SidesNaming by AnglesTicks into EquationsThe Longest SideRecap: Sorting Triangles4Area of a Triangle
Half the RectangleHeight Is Not a SideRight Triangle AreaFinding the HeightAny Side Can Be the BaseRecap: Area2The Triangle Inequality
Can Three Sticks Meet?Exactly ReachingThe Range of the Third SideThe FormulaRecap: The Third Side5The Pythagorean Theorem
Squares on the SidesFinding the HypotenusePythagorean TriplesWhen It Is Not WholeRecap: The Hypotenuse3Perimeter
Around the EdgeA Missing SideIsosceles PerimeterSides as ExpressionsRecap: Around the Edge6Finding a Leg
Working BackwardsThe LadderWhich Side Is c?A Leg That Is Not WholeRecap: Finding a Leg