See complete Problem 104

Triangle, Angles, Midpoint, Congruence. Level: High School, SAT Prep, College geometry

Post your solutions or ideas in the comments.

## Monday, May 19, 2008

### Geometry Problem 104

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## Monday, May 19, 2008

###
Geometry Problem 104

## Link List

Online Geometry theorems, problems, solutions, and related topics.

See complete Problem 104

Triangle, Angles, Midpoint, Congruence. Level: High School, SAT Prep, College geometry

Post your solutions or ideas in the comments.

Subscribe to:
Post Comments (Atom)

Let I be an incenter fo the trinagle ABD. Then

ReplyDeleteangle of IBC = angle of IDC=90 degree and so the 4-points B,I,D,C on a circle, that is,

the quadrilateral IDCB is a cycle.

Hence, angle of BCD = alpha +beta and

angle of 180- alpha -beta= 90+((angle of )/2).

Furthermore,

angle of DIC=90-alpha

=(angle of A)/2 +(angle of IDA) and hence

the points A, I, E, C are on the same line, that is,

the points A, I, E, C are on the extended diagonal of the cicumcircle of the

quadrilateral IDCB. and so

x= angle of AEB=90+beta-alpha.

2009.9.20

From seoul, South Korea.

Bae deok rak

bdr@korea.com

This solution to problem 104 follows the same idea of Bae Deok Rak. I think that I’ve drawn it up in a clearer way.

ReplyDeleteLet I be the incenter of triangle ABD. Then ang(IBC) = ang(IDC) = 90, the quadrilateral BCDI is cyclic, ang(BCI) = beta and ang(DCI) = alpha.

We must prove that lines AC and IC are the same line.

In triangle ADI, ang(AID) = 180 – A/2 – beta, and in triangle DIC, ang(DIC) = 90 – alpha. So, ang(AID) + ang(DIC) =

= 270 – A/2 – alpha – beta (1).

In triangle ABD, A = 180 – 2.alpha – 2.beta, or A/2 = 90 – alpha – beta. Therefore, in (1), we get ang(AID) + ang(DIC) =

= 270 – (90 – alpha – beta) – alpha – beta = 180.

This conclusion shows that A, I, E and C are collinear points.

Finally, in triangle BCE, we have

ang(AEB) = ang(DBC) + ang(BCI) = (90 – alpha) +beta = 90 + beta – alpha.

BC and DC are external bisectors of Tr ABD hence CA is the internal bisector and the result follows.

ReplyDeletesumith peiris

Moratuwa

Sri Lanka