Lesson · Grade 6 · Mathematics
Area of a Triangle: Half of a Parallelogram
- Grade 6
- Mathematics
- 1 min 19 s
- British Columbia
- BC6-MA-31
A triangle with a base of 8 cm and a height of 5 cm is copied, given a half turn and fitted against its sloping side, making a parallelogram with the same base and height, whose area is 8 × 5 = 40 cm². The triangle is exactly half of it, 40 ÷ 2 = 20 cm², which is checked by 20 + 20 = 40, and the video ends on the rule: a triangle's area is base × height ÷ 2.
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In one sentence
The question this lesson answers
What is the area of this triangle?
Its base is 8 centimetres long.
Transcript
What the lesson says
What is the area of this triangle?
Its base is 8 centimetres long.
Its height is 5 centimetres, measured at a right angle to the base.
Now make an exact copy of the triangle.
Give the copy a half turn, then slide it into place.
Together, the two triangles make a parallelogram.
The parallelogram has the same base and the same height as the triangle.
The area of a parallelogram is base times height.
8 times 5 is 40, so the parallelogram is 40 square centimetres.
The two triangles are identical, so each one is exactly half of the parallelogram.
40 divided by 2 is 20, so our triangle is 20 square centimetres.
Check: 20 plus 20 is 40, the whole parallelogram.
So the area of a triangle is base times height divided by 2.
Here, 8 times 5 divided by 2 is 20 square centimetres.
Remember: a triangle is half a parallelogram, so its area is base times height divided by 2.
Curriculum
Where this fits
- British Columbia, Grade 6
Mathematics, Geometry and measurement: area of triangles, parallelograms, and trapezoidsBC6-MA-31
Previous: Area of a Parallelogram: Cut and Slide into a RectangleNext: Area of a Trapezoid: Two Copies Make a Parallelogram
The outcomes and strands follow the British Columbia curriculum.
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