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Korea North TST 2013 2p

 geometry problems from North Korean Team Selection Tests (TST) with aops links in the names


2013 only

2013 North Korea TST p1

The incircle of a non-isosceles triangle ABC with the center I touches the sides BC, CA, AB at A_1 , B_1 , C_1 respectively. The line AI meets the circumcircle of ABC at A_2 . The line B_1 C_1 meets the line BC at A_3 and the line A_2 A_3 meets the circumcircle of ABC at A_4 (\ne A_2 ) . Define B_4 , C_4 similarly. Prove that the lines AA_4 , BB_4 , CC_4 are concurrent.

2013 North Korea TST p5

The incircle \omega of a quadrilateral ABCD touches AB, BC, CD, DA at E, F, G, H , respectively. Choose an arbitrary point   X on the segment AC inside \omega . The segments XB, XD meet \omega at I, J respectively. Prove that FJ, IG, AC are concurrent.


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