Maths Olympiad Prep

Library / /1 of 2

Geometry Difficulty 5.5 AIME, harder Prove it Argentina

Find the angles of a convex quadrilateral ABCDABCD such that ABD^=29\hat{ABD} = 29^\circ, ADB^=41\hat{ADB} = 41^\circ, ACB^=82\hat{ACB} = 82^\circ and ACD^=58\hat{ACD} = 58^\circ.

Solution

We have BAD^=180(29+41)=110\hat{BAD} = 180^\circ - (29^\circ + 41^\circ) = 110^\circ, BCD^=82+58=140\hat{BCD} = 82^\circ + 58^\circ = 140^\circ. Consider the circumcircle γ\gamma of BCD\triangle BCD. Since BAD^+BCD^>180\hat{BAD} + \hat{BCD} > 180^\circ, point AA is interior to γ\gamma.

Extend CACA beyond AA to meet γ\gamma at EE. By inscribed angles EBD^=ECD^=ACD^=58\hat{EBD} = \hat{ECD} = \hat{ACD} = 58^\circ, EDB^=ECB^=ACB^=82\hat{EDB} = \hat{ECB} = \hat{ACB} = 82^\circ.

Given that ABD^=29\hat{ABD} = 29^\circ, ADB^=41\hat{ADB} = 41^\circ we obtain that BABA and DADA are bisectors of EBD^\hat{EBD} and EDB^\hat{EDB} respectively.

Hence AA is the incenter of triangle BDEBDE, implying that EAEA is the bisector of BED^\hat{BED}.

From the cyclic quadrilateral BCDEBCDE we have
BED^=180BCD^=BEC^=12BED^=20 and analogously \hat{BED} = 180^\circ - \hat{BCD} = \hat{BEC} = \frac{1}{2} \hat{BED} = 20^\circ \text{ and analogously}
DBC^=20. In conclusion, ABC^=29+20=49,ADC^=41+20=61. \hat{DBC} = 20^\circ. \text{ In conclusion, } \hat{ABC} = 29^\circ + 20^\circ = 49^\circ, \hat{ADC} = 41^\circ + 20^\circ = 61^\circ.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.