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In triangle PQR, \(\angle P = 55^{\circ}\) and QR = 18 cm. In which of the following cases, \(\Delta PQR\) can be an obtuse scalene triangle ?

  • A.\(\angle R = 25^{\circ}\) and PQ = 18 cm
  • B.\(\angle R = 15^{\circ}\) and PR \(\gt\) 18 cm
  • C.\(\angle R = 65^{\circ}\) and PQ \(\gt\) 18 cm
  • D.\(\angle R = 35^{\circ}\) and PR = 18 cm

Answer: B

The triangle can be obtuse and scalene when \(\angle R = 15^{\circ}\) and PR \(\gt\) 18 cm.

With \(\angle P = 55^{\circ}\) and \(\angle R = 15^{\circ}\), \(\angle Q = 110^{\circ}\), an obtuse angle. The longest side is opposite the largest angle, so PR (opposite Q) is longer than QR = 18 cm, as stated, and all three angles differ, so the sides do too.

With \(\angle R = 25^{\circ}\), PQ = QR would force \(\angle R = \angle P\), a contradiction. \(\angle R = 65^{\circ}\) gives \(\angle Q = 60^{\circ}\), not obtuse. \(\angle R = 35^{\circ}\) gives a right angle at Q.

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