Will Angle Chasing Work?

Geometry Level 4

In the figure B D A = C D B = 5 0 , D A C = 2 0 , C A B = 8 0 \angle BDA=\angle CDB=50^\circ,\angle DAC=20^\circ ,\angle CAB=80^\circ

If B C A = x , D B C = y \angle BCA=x^\circ,\angle DBC=y^\circ

Then find the sum of the digits of the number x y x^{y} .


The answer is 36.

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4 solutions

Steven Yuan
Mar 22, 2015

We have A B D = 3 0 \angle ABD = 30^{\circ} and A C D = 6 0 \angle ACD = 60^{\circ} . Let the angle bisector of A C D \angle ACD intersect B C BC at P P . Then, A C P = A B P = 3 0 \angle ACP = \angle ABP = 30^{\circ} , so A B C P ABCP is cyclic. Since C P CP and B D BD are both angle bisectors of A C D \triangle ACD , they intersect at the incenter of A C D \triangle ACD , which is P P . Thus, C A P = C B D = 1 0 \angle CAP = \angle CBD = 10^{\circ} and B C A = 6 0 \angle BCA = 60^{\circ} . Finally, x y = 6 0 10 x^y = 60^{10} , which has digit sum 36 \boxed{36} .

Great job! I actually used sine rule. But your solution must be the best way to do it.

Raghav Vaidyanathan - 6 years, 2 months ago
Joel Tan
Apr 17, 2015

B is the intersection of the external angle bisector of angle CAD and the angle bisector of angle ADC. Thus B is the centre of the excircle opposite D. Thus BC is the external angle bisector of angle ACD, and angle BCA is 60°. Thus

x = 60 , y = 10 , x y = 604661760000000000 x=60, y=10, x^{y}=604661760000000000 which has sum of digits 36.

Sakanksha Deo
Mar 23, 2015

Draw angle bisector of B C D \angle{BCD} meeting BD at E

Now.

Since A B E = A C E = 30 \angle{ABE} = \angle{ACE} = 30

Therefore ABCE is cylic

Also since angle bisectors of D \angle{D} and A C D \angle{ACD} meet at E.

Therefore E is the incentre of A D C \triangle ADC

So,

F A E = 10 \angle{FAE} = 10

This gives,

x = ( 180 90 ) 30 = 60 x = ( 180 - 90 ) - 30 = 60

So

y = 10 y = 10

Now,

x y = 6 0 10 = 6 10 × 1 0 10 = 60466176 × 1 0 10 x^y = 60^{10} = 6^{10} \times 10^{10} = 60466176 \times 10^{10}

Therefore ,

Sum of digits = 36 \boxed{36}

John Wyatt
Mar 22, 2015

AXD = 180 - (DAC + BBA) = 110 (Angles in a triangle make 180)

BXC = AXD = 110 (Opposite Angles)

AXB = 180-BXC = 70 (Angles in a straight line)

CXD = AXB = 70 (Opposite Angles)

ABD = 180 - (CAB + AXB) = 30 (Angles in a triangle make 180)

I can see 2 methods for getting reming 2 angles :

Construction or Sin Rule (Both require a random length for starting point which is fine as it is only angles we care about)

I went with construction and got BCA = 60 & DBC = 10

60^10 = 6^10 * 10^10

6^10 * 10^10 = 60466176 * 10000000000

==> 6+0+4+6+6+1+7+6 +(10*0)= 36

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