We use the following formula to work out the area of a rectangle:
We need to know the base length and the perpendicular height from the base to the top.
Area of rectangle = base × height (b x h)
Let's have a go:
What is the area of a rectangle with the following measurements?
Base = 9 cm
Height = 7 cm
9 cm x 7 cm = 63 cm²
We can use this formula to work out the area of a triangle.
Try this with a piece of paper.
Start with a rectangle.
Cut off a right-angled triangle from the left-hand side and separate the two pieces.
The area of the two pieces together will be the same as the original rectangle.
Cut off a right-angled triangle from the right-hand side to form a triangle in the middle, and separate the three pieces.
The area of the three pieces will still be the same as the original rectangle.
Now look carefully at the pieces marked A and B below.
Notice that the areas match.
A matches A and B matches B.
So, the area of the triangle in the middle is half the area of the rectangle.
Area of Triangles
We need to know the base length and the perpendicular height from the base to the apex. (not the slant height)
Area of triangle = ½ base × height
or
Area of triangle = base × height ÷ 2
Example
Calculate the area of this triangle in cm2
Answer
If we turn our heads to the left, the base length is 12 cm and the perpendicular height is 6 cm.
We do not need to know the slant height of 10 cm to use the formula.
Area of triangle = ½ base × height
Area = ½ × 12 × 6 = 6 × 6 = 36 cm2
or
Area of triangle = base × height ÷ 2
Area = 12 x 6 ÷ 2 = 72 ÷ 2 = 36 cm2
Now, why don't you have a go at calculating the area of some triangles?