Question

The perimeter of a triangle is 29.6 centimetre and two of the sides are of length 11.8 centimetres and 9.4 centimetres. Find the length of the third side.

Answer 1

Perimeter of the triangle = 29.6 cm

The lengths of two sides of the triangle are 11.8 cm and 9.4 cm.

These lengths can also be written as:

29.6 cm = 29 cm + 6 mm

11.8 cm = 11 cm + 8 mm

9.4 cm = 9 cm + 4 mm

We know that the perimeter of a triangle is equal to the sum of the lengths of all its sides.

Hence, the length of the third side can be calculated by subtracting the sum of the lengths of the other two sides from the perimeter of the triangle.

∴ Length of the third side = (29 cm + 6 mm) − {(11 cm + 8 mm) − (9 cm + 4 mm)}

The addition of the sides:

		cm		mm
		11		8
+		9		4
		21		2

The subtraction of the sum of the lengths of the other two sides from the perimeter:

		cm		mm
		29		06
−		21		02
		08		04

Therefore, the length of the third side is 8 cm and 4 mm, i.e. 8.4 cm.