Theorem 15: Converse of Pythagoras’s theorem – If the square of one side of a triangle is the sum of the squares of the other two, then the angle opposite the fi rst side is a right angle.
When the three sides of a triangle satisfy c2 = a2 + b2 , then the triangle is right-angled.
Investigating whether a triangle is right-angled 1
3
Label the sides of the triangle a, b and c. Remember c is the longest side. Evaluate each side of the formula.
2 4
Substitute the values of a, b and c in the Pythagoras formula.
If the left-hand side (LHS) = the right-hand side (RHS) the triangle is right-angled.
Worked example 3 Investigate whether the following triangles are right-angled. (i)
L 15 12 M 9
Solution (i)
N
(ii) U 11 W
7
V 6
Practice questions 22.3
1. Identify the hypotenuse in each of the following triangles. Justify your answer in each case. (i)
X (ii) T S (iii) P Z (ii) Y 8 cm (iii) R 8 m (iv) (v) Q
2. Find the length of the hypotenuse in each of the following right-angled triangles. (i)