Source: Previous Question Papers

In the adjoining △PQR\triangle PQR, ray PA and ray QB are the bisectors of ∠P\angle P and ∠Q\angle Q respectively. If ∠R=70∘\angle R=70^\circ. Find m∠PMQm \angle PMQ.

Question Illustration

Options

Option A

120°

Option B

130°

Option C is correct

125°

Option D

135°

Explanation

In △PQR\triangle PQR, the sum of angles is 180∘180^\circ. Given ∠R=70∘\angle R=70^\circ, we have ∠P+∠Q=180∘−70∘=110∘\angle P + \angle Q = 180^\circ - 70^\circ = 110^\circ. Since PA and QB are angle bisectors, in △PMQ\triangle PMQ, ∠QPM=12∠P\angle QPM = \frac{1}{2}\angle P and ∠PQM=12∠Q\angle PQM = \frac{1}{2}\angle Q. The sum of angles in △PMQ\triangle PMQ is 180∘180^\circ, so ∠PMQ+∠QPM+∠PQM=180∘\angle PMQ + \angle QPM + \angle PQM = 180^\circ. This simplifies to ∠PMQ+12(∠P+∠Q)=180∘\angle PMQ + \frac{1}{2}(\angle P + \angle Q) = 180^\circ. Substituting the sum of ∠P\angle P and ∠Q\angle Q, we get ∠PMQ+12(110∘)=180∘\angle PMQ + \frac{1}{2}(110^\circ) = 180^\circ. Thus, ∠PMQ+55∘=180∘\angle PMQ + 55^\circ = 180^\circ, which means ∠PMQ=180∘−55∘=125∘\angle PMQ = 180^\circ - 55^\circ = 125^\circ. Alternatively, the angle formed by two angle bisectors is given by 90∘+12∠R=90∘+12(70∘)=90∘+35∘=125∘90^\circ + \frac{1}{2}\angle R = 90^\circ + \frac{1}{2}(70^\circ) = 90^\circ + 35^\circ = 125^\circ.