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The sides of four triangles are given in the options.

Which of them forms a right triangle ?

  • A.(i) 20 cm, 22 cm, 24 cm
  • B.(ii) 15 cm, 32 cm, 37 cm
  • C.(iii) 11 cm, 60 cm, 61 cm
  • D.(iv) 19 cm, 40 cm, 41 cm

Answer: C

The right triangle is (iii), with sides 11 cm, 60 cm and 61 cm.

By the converse of Pythagoras' theorem, a triangle is right-angled when the square of the longest side equals the sum of the squares of the other two:

\[ \begin{aligned} 11^{2} + 60^{2} &= 121 + 3600 = 3721 \\ 61^{2} &= 3721 \end{aligned} \]

The others fail: \(20^{2} + 22^{2} = 884 \ne 576\); \(15^{2} + 32^{2} = 1249 \ne 1369\); \(19^{2} + 40^{2} = 1961 \ne 1681\). The familiar triple 9, 40, 41 has 9, not 19, so (iv) is a near miss.

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