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Mongolian Mathematical Olympiad

Geometry Difficulty 5.8 AIME, harder Prove it Mongolia

If ∠CBA=90∘\angle CBA = 90^\circ, ∠BAP=∠PAR=∠RAC\angle BAP = \angle PAR = \angle RAC, ∠BCQ=∠QCR=∠RCA\angle BCQ = \angle QCR = \angle RCA, ∠QRC=142∘\angle QRC = 142^\circ then find the angle ∠BAC\angle BAC.

Figure 1

Solution

Denote ∠BAP=∠PAR=∠RAC=α\angle BAP = \angle PAR = \angle RAC = \alpha. Then ∠BAC=3α\angle BAC = 3\alpha. Denote ∠BCQ=∠QCR=∠RCA=γ\angle BCQ = \angle QCR = \angle RCA = \gamma. Then ∠ACB=3γ\angle ACB = 3\gamma. Since ∠ABC=90∘\angle ABC = 90^\circ, ∠ACB+∠BAC=180∘−90∘=90∘⇒3α+3γ=90∘\angle ACB + \angle BAC = 180^\circ - 90^\circ = 90^\circ \Rightarrow 3\alpha + 3\gamma = 90^\circ and α+γ=30∘\alpha + \gamma = 30^\circ.

△ACR\triangle ACR : ∠ARC=180∘−(α+β)=150∘\angle ARC = 180^\circ - (\alpha + \beta) = 150^\circ. Let (CQ)∩(AP)=S(CQ) \cap (AP) = S.
Since ARAR, CRCR bisectors of the △ASC\triangle ASC, SRSR also bisector. ∠ASR=∠CSR=60∘⇒∠ASQ=60∘\angle ASR = \angle CSR = 60^\circ \Rightarrow \angle ASQ = 60^\circ. It implies △ASR=△ASQ\triangle ASR = \triangle ASQ. Hence SQ=SRSQ = SR and △SQR\triangle SQR is isosceles. Therefore ∠SQR=∠SRQ=30∘\angle SQR = \angle SRQ = 30^\circ.
△CRQ\triangle CRQ : γ=180∘−(142∘+30∘)=8∘⇒α+γ=30∘\gamma = 180^\circ - (142^\circ + 30^\circ) = 8^\circ \Rightarrow \alpha + \gamma = 30^\circ and α=22∘\alpha = 22^\circ. Thus
∠BAC=3α=66∘\angle BAC = 3\alpha = 66^\circ.

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