Suppose $ABCD$ is a cyclic quadrilateral with $BC = CD$. Let $\omega$ be the circle with center $C$ tangential to the side $BD$. Let $I$ be the centre of the incircle of triangle $ABD$. Prove that the straight line passing through $I$, which is parallel to $AB$, touches the circle $\omega$.
Problem
Source: Czech - Polish - Slovak Match 2013: P1
Tags: geometry, geometric transformation, homothety, LaTeX, cyclic quadrilateral, geometry unsolved
24.05.2014 17:22
Do you mean tangent by touches the circle $\omega$ cause I think it cannot be a tangent(only in a specific case).
24.05.2014 20:36
I have some proof(that it intersects) which I hope is correct. I am giving a partial solution right now and will complete when I have time. Obviously $AIC$ is a straight line. Also it is pretty well known $CD=CI=CB$. Let the circle passing through $D,I,B$ with $C$ centre be $\omega_1$ Now consider the homothety $H_C:I \to A$ that is centred at $C$ which takes $I$ to $A$. We will show $H_C:\omega \to \omega_1$.
24.05.2014 23:50
consider$I_e$ the excenter ,T the projection of C on the parallel to AB through I X= $\frac{R-r}{2},tan \frac{A}{2}=\frac{r}{s-a}=\frac{R}{s}=\frac{r'}{\frac{a}{2}}$,where[a=BD,s=$ \frac{BD+AB+AD}{2}$] the consequence is obvious
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25.05.2014 15:50
Firstly, $ A,I,C $ are colinear and $ BC=IC=DC $ And let $ R $ be the tangent point of $ \omega $ with $ BD $ . Clearly, $ BR=RD $ and now let $ Q $ be the pedal of projection from $ C $ to the straight line through $ I $ and parallel to $ AB $ . Now, consider, $ \triangle IQC $ and $ \triangle BRC $ : We have: $ BC=IC $ , $ \angle CBR=\angle QIC=\angle A/2 $ . And now as they are right angled so, $ \angle BCR=\angle ICQ $ .so they are congruent and finally, $ CQ=CR $
29.05.2014 10:31
I solved this problem,but i don't know Latex inthedarkness wrote: Firstly, $ A,I,C $ are colinear and $ BC=IC=DC $ And let $ R $ be the tangent point of $ \omega $ with $ BD $ . Clearly, $ BR=RD $ and now let $ Q $ be the pedal of projection from $ C $ to the straight line through $ I $ and parallel to $ AB $ . Now, consider, $ \triangle IQC $ and $ \triangle BRC $ : We have: $ BC=IC $ , $ \angle CBR=\angle QIC=\angle A/2 $ . And now as they are right angled so, $ \angle BCR=\angle ICQ $ .so they are congruent and finally, $ CQ=CR $
29.05.2014 11:19
So your method is same as me? Congrats!
29.05.2014 11:22
So your method is same as me? Congrats!
29.05.2014 15:15
Let $IX$ be the tangent from $I$ to the circle $\omega$ and $X$ lies on $\omega$ and let $\omega$ touches $BD$ at $M$. The equality $CB = CI = CD$ is well-known and easily proven, then the triangles $\triangle{CXI} \cong ruent \triangle{CMB}$. So, $\angle{CAB} = \angle{CBM} = \angle{CIX}$. So, $IX$ is parallel to $AB$. Thus, $IX$ is the line through $I$ and parallel to $AB$.