∠ B D A = ∠ C D B = 5 0 ∘ , ∠ D A C = 2 0 ∘ , ∠ C A B = 8 0 ∘
In the figureIf ∠ B C A = x ∘ , ∠ D B C = y ∘
Then find the sum of the digits of the number x y .
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Great job! I actually used sine rule. But your solution must be the best way to do it.
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 = 6 0 , y = 1 0 , x y = 6 0 4 6 6 1 7 6 0 0 0 0 0 0 0 0 0 0 which has sum of digits 36.
∠ B C D meeting BD at E
Draw angle bisector ofNow.
Since ∠ A B E = ∠ A C E = 3 0
Therefore ABCE is cylic
Also since angle bisectors of ∠ D and ∠ A C D meet at E.
Therefore E is the incentre of △ A D C
So,
∠ F A E = 1 0
This gives,
x = ( 1 8 0 − 9 0 ) − 3 0 = 6 0
So
y = 1 0
Now,
x y = 6 0 1 0 = 6 1 0 × 1 0 1 0 = 6 0 4 6 6 1 7 6 × 1 0 1 0
Therefore ,
Sum of digits = 3 6
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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We have ∠ A B D = 3 0 ∘ and ∠ A C D = 6 0 ∘ . Let the angle bisector of ∠ A C D intersect B C at P . Then, ∠ A C P = ∠ A B P = 3 0 ∘ , so A B C P is cyclic. Since C P and B D are both angle bisectors of △ A C D , they intersect at the incenter of △ A C D , which is P . Thus, ∠ C A P = ∠ C B D = 1 0 ∘ and ∠ B C A = 6 0 ∘ . Finally, x y = 6 0 1 0 , which has digit sum 3 6 .