olymmath

HK-PC 2020

Hong Kong Preliminary Selection Contest and TST 2020.

Other problems

Problem 5

In ΔABC\Delta ABC , let DD be a point on side BCBC . Suppose the incircle ω1\omega_1 of ΔABD\Delta ABD touches sides ABAB and ADAD at E,FE,F respectively, and the incircle ω2\omega_2 of ΔACD\Delta ACD touches sides ADAD and ACAC at F,GF,G respectively. Suppose the segment EGEG intersects ω1\omega_1 and ω2\omega_2 again at PP and QQ respectively. Show that line ADAD , tangent of ω1\omega_1 at PP and tangent of ω2\omega_2 at QQ are concurrent.

Problem 6

For a sequence with some ones and zeros, we count the number of continuous runs of equal digits in it. (For example the sequence 011001010011001010 has 77 continuous runs: 0,11,00,1,0,1,00,11,00,1,0,1,0 .) Find the sum of the number of all continuous runs for all possible sequences with 20192019 ones and 20192019 zeros.