Hunting In December.

Geometry Level 4

Let A B C \triangle ABC be an acute triangle with incentre I I such that A B C = 3 5 \angle ABC = 35^{\circ} . Suppose that A I + A C = B C AI + AC = BC . Let P P be the intersection of the tangents at B , C B, C to A B C \odot ABC and let Q Q be the intersection of the line through P P parallel to A C AC with A B AB .

Find A Q C \angle AQC in degree measures.


The answer is 40.

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1 solution

Let D D be a point on B C \overline{\rm BC} such that C D \overline{\rm CD} = = C A \overline{\rm CA} . Draw A D \overline{\rm AD} , A I \overline{\rm AI} , B I \overline{\rm BI} & C I \overline{\rm CI} .

B D \overline{\rm BD} = = B C \overline{\rm BC} - C D \overline{\rm CD} = = B C \overline{\rm BC} - A C \overline{\rm AC} = = A I \overline{\rm AI}

A D B \angle {ADB} = = A C D \angle {ACD} + + D A C \angle {DAC} = = A C D \angle {ACD} + + 90 90 - A C D 2 \frac{\angle {ACD}}{2} = = 90 90 + + A C D 2 \frac{\angle {ACD}}{2} = = A I B \angle {AIB}

In triangles A I B \bigtriangleup AIB and B D A \bigtriangleup BDA , A I \overline{\rm AI} = = B D \overline{\rm BD} , A I B \angle {AIB} = = A D B \angle {ADB} & A B \overline{\rm AB} is common to them. Therefore, A I B \bigtriangleup AIB \cong B D A \bigtriangleup BDA by the S S A S-S-A criterion of congruence.

Hence, B A I \angle {BAI} = = A B D \angle {ABD} = = 3 5 35^{\circ} \Rightarrow B A C \angle {BAC} = = 2 2 B A I \angle {BAI} = = 70 70

A C B \angle {ACB} = = 180 70 35 180-70-35 = = 7 5 75^{\circ}

Let O O be the circumcentre of A B C \bigtriangleup ABC . Draw O B \overline{\rm OB} and O C \overline{\rm OC} . O B \overline{\rm OB} \perp B P \overline{\rm BP} and O C \overline{\rm OC} \perp C P \overline{\rm CP} . \Rightarrow B O C P BOCP is a cyclic quadrilateral.

B P C \bigtriangleup BPC is isosceles. \Rightarrow B C P \angle {BCP} = = 180 B P C 2 \frac{180-\angle {BPC}}{2} = = B O C 2 \frac{\angle {BOC}}{2} = = B A C \angle {BAC} .

Observe that, B Q P \angle {BQP} = = B A C \angle {BAC} = = B C P \angle {BCP} , which points out that B Q C P BQCP is a cyclic quadrilateral.

Hence, A Q C \angle {AQC} = = 180 B Q C 180-\angle{BQC} = = B P C \angle {BPC} = = 180 B O C 180-\angle{BOC} = = 180 2 B A C 180-2\angle{BAC} = = 180 2 × 70 180-2{\times}70 = = 40 \boxed{\textbf{40}} .

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