This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

Tuesday, March 03, 2009

Calculating the Area of a Triangle

Area of a triangle = ½ x base x height

A very common mistake students make is extending the base outside the triangle.
Example -
Question – Find the area of the shaded triangle.

Common Mistake (Wrong solution)

A common mistake students make in the above question is:-

Area of triangle = ½ x base x height
= ½ x 6cm x 3cm = 9 square cm (wrong)

The base should be 4 cm and not 6 cm. The question asks you to calculate the area of the SHADED triangle. The base of the shaded triangle is 4 cm.

If you put the base as 6 cm, you will calculate the area of the WHOLE figure as shown below.

The correct answer is as follows:

Area of triangle = ½ x base x height
= ½ x 4cm x 3cm = 6 square cm (correct answer)

In order to avoid making the mistake above, always remember that the base of the triangle must never extend outside the triangle.


chicchicbaby said...

I thought height must be at right angle with the base? If thats the case, the height for the shaded triangle cannot be 3cm. Shouldnt we calculate the area of the entire triangle, then less off the area of the small right angle triangle?? Pls adv, I may be wrong.

Anonymous said...

The height is 3 cm.

You can try to calculate using your method. You will realise that the answer is the same. So why go through the trouble of using Big Right-angled Triangle - Small Right-angled Triangle.

Anonymous said...

I thought it should be:

a) 1/2 x 6 x 3 = 9 cm 2
b) 1/2 x 2x 3 = 3 cm 2
9 - 6 = 3 cm 2