Just an Angle!

Geometry Level 3

Find P C . PC.

Note: This length can be expressed as A B \sqrt{\dfrac{A}{B}} , where A , B A,B are coprime positive integers. Submit the value of A + B A+B as your answer.


The answer is 79.

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9 solutions

Jessica Wang
Sep 27, 2015

Extend P B PB and A C AC , label the point of intersection as E E .

We have A E P = 3 0 \angle AEP=30^{\circ} , C E = 6 , A E = 8 \Rightarrow CE=6, \; AE=8 , A P = 8 3 3 \Rightarrow AP=\frac{8\sqrt{3}}{3} , P C = 2 2 + ( 8 3 3 ) 2 = 2 57 3 = 4 × 57 9 = 76 3 \Rightarrow PC=\sqrt{2^{2}+\left ( \frac{8\sqrt{3}}{3} \right )^{2}}=\frac{2\sqrt{57}}{3}=\sqrt{\frac{4\times 57}{9}}=\sqrt{\frac{76}{3}} .

Thinking outside the box. Short and sweet. Excellent approach.

James O'Reilly - 3 years, 2 months ago
Chew-Seong Cheong
Sep 22, 2015

Let P C = x PC = x and A P C = θ \angle APC = \theta , then using sine rule, we have:

x sin 9 0 = x = 2 sin θ = 3 sin ( 6 0 θ ) 2 ( sin ( 6 0 θ ) ) = 3 sin θ 2 ( 3 2 cos θ 1 2 sin θ ) = 3 sin θ 3 cos θ sin θ = 3 sin θ 3 cos θ = 4 sin θ 3 cos 2 θ = 16 sin 2 θ 3 ( 1 sin 2 θ ) = 16 sin 2 θ 3 = 19 sin 2 θ sin θ = 3 19 x = 2 sin θ = 2 3 19 = 76 3 A + B = 76 + 3 = 79 \begin{aligned} \frac{x}{\sin 90^\circ} = x & = \frac{2}{\sin{\theta}} = \frac{3}{\sin{(60^\circ - \theta)}} \\ \Rightarrow 2(\sin{(60^\circ - \theta)}) & = 3\sin{\theta} \\ 2\left(\frac{\sqrt{3}}{2}\cos \theta - \frac{1}{2} \sin \theta\right) & = 3 \sin \theta \\ \sqrt{3}\cos \theta - \sin \theta & = 3 \sin \theta \\ \sqrt{3}\cos \theta & = 4 \sin \theta \\ 3 \cos^2 \theta & = 16 \sin^2 \theta \\ 3 (1 - \sin^2 \theta ) & = 16 \sin^2 \theta \\ 3 & = 19 \sin^2 \theta \\ \Rightarrow \sin \theta & = \sqrt{\frac{3}{19}} \\ \Rightarrow x & = \frac{2}{\sin \theta} = \frac{2}{\sqrt{\frac{3}{19}}} = \sqrt{\frac{76}{3}} \\ \\ \Rightarrow A + B & = 76 + 3 = \boxed{79} \end{aligned}

verı got...

Soner Karaca - 5 years, 7 months ago
Tanishq Varshney
Sep 19, 2015

I used sine rule in the very first step

Brilliant! This is a usual solution. Can you think of other solutions? Shorter ones?

Satyajit Mohanty - 5 years, 8 months ago
Yuen Ho Lam
Sep 20, 2015

Extend the line BC to intersect extension of line AP at point D.

DAC=30 degrees

CD=2/sin(60)=4

BP=7tan(30)=7/sqrt(3)

CP^2=BP^2+BC^2

CP=sqrt(76/3)

That's what I have used. It's a simple problem though.

Eddy Li - 5 years, 8 months ago

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good job Eddy!

Alex Zheng - 5 years, 8 months ago

No gimmicks ;)

Alan Yan
Sep 20, 2015

P = ( 0 , 0 ) P = (0,0) C = ( x , 3 ) C = (x, 3) A P : y = tan π 3 x 3 x y = 0 AP : y = \tan \frac{\pi}{3} x \implies \sqrt{3}x - y = 0 3 x 3 3 2 + 1 2 = 3 x 3 2 = 2 3 x 3 = 4 \frac{ |\sqrt{3}x - 3| }{\sqrt{ \sqrt{3}^2 + 1^2} } = \frac{|\sqrt{3}x - 3|}{2} = 2 \implies |\sqrt{3}x - 3| = 4 x = 7 3 Other solution is extraneous \implies x = \frac{7}{\sqrt{3}} \text{ Other solution is extraneous} C P = x 2 + 9 = 49 3 + 9 = 76 3 76 + 3 = 79 CP = \sqrt{x^2 + 9} = \sqrt{\frac{49}{3} + 9} = \sqrt{\frac{76}{3}} \implies 76 + 3 = \boxed{ 79 }

Nice use of co-ordinate geometry!

Satyajit Mohanty - 5 years, 8 months ago
Reynan Henry
Sep 20, 2015

Let the radius be x then x²+x²-2x.xcos120 = 4 + 9 - 2.2.3.cos120. Then x = sqrt(19/3). PC is diameter so PC = 2x = sqrt(76/3). Sry cant use latex im confused

Wouter Dobbelaere
Oct 23, 2020

C can be found as intersection of 2 lines:

  1. Line1 parallel with AP at distance 2: y=sqrt(3).x-4
  2. Line2 : y=3

So the x coordinate of C is 7/sqrt(3)

-> PC=sqrt(3^2 + (7/sqrt(3))^2)=sqrt(76/3)

Shourya Pandey
Feb 23, 2017

Here I present a proof using coordinate geometry.

Let P = ( 0 , 0 ) P= (0,0) and B = ( b , 0 ) B=(b,0) , where b > 0 b>0 . Then O = ( b , 3 ) O= (b,3) . Now

S l o p e O A = 1 S l o p e O B = 1 t a n 6 0 = t a n 3 0 Slope OA = \frac{-1}{Slope OB} = \frac{-1}{tan 60^\circ} = - tan 30^\circ ,

and O A = 2 OA=2 with A A to the left of O O so the point A = ( b 2 c o s 3 0 , 3 + 2 s i n 3 0 ) A = (b-2cos 30^\circ , 3+ 2 sin 30^\circ ) = ( b 3 , 4 ) = (b- \sqrt{3} , 4) .

Now, the slope of P A PA is t a n 6 0 = 3 tan 60^\circ = \sqrt{3} , so

4 b 3 = 3 \frac {4}{b-\sqrt{3}} = \sqrt{3} , giving b = 7 3 b= \frac{7}{\sqrt{3}} .

Therefore P C = b 2 + 9 = 49 3 + 9 = 76 3 PC= \sqrt {b^{2} + 9} = \sqrt{\frac{49}{3} + 9} = \sqrt {\frac{76}{3}} , and so

A + B = 76 + 3 = 79 A+B = 76+ 3= 79 .

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