1500 followers problem! For my friend Xuming!

Geometry Level 5

In Δ A B C \Delta ABC , m B = 4 5 m\angle B = 45^\circ , m C = 5 4 m\angle C=54^\circ . Point O O is the circumcenter. Let O E A C , O F A B \overline{OE}\perp \overline{AC} \ , \ \overline{OF}\perp \overline{AB} . If the ratio O E O F \dfrac{OE}{OF} can be expressed as a b b \dfrac{a}{\sqrt{b-\sqrt{b}}} where a , b Z + a,b \in \mathbb{Z}^+ and b b is square free,

Find the value of 1500 a + b \displaystyle \left \lfloor\frac{1500}{a+b} \right \rfloor .


The answer is 214.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

4 solutions

Richard Zhou
Sep 5, 2015

We first construct segments O A , O B , \overline { OA } ,\overline { OB } , and O C \overline { OC } . Since O is the circumcenter, we know that these three segments have the same length, and triangles A O B , A O C , AOB, AOC, and B O C BOC are isosceles. Thus, E E and F F are bisectors of sides A C AC and A B AB , respectively.

We then assign the measure of B A O \angle BAO as α \alpha , and from here we angle-chase a little bit until we reach A B O = 72 α \angle ABO=72-\alpha . Since A B O = B A O = α = 72 α \angle ABO=\angle BAO=\alpha =72-\alpha , α = 36 \alpha =36 . Thus, F A O = α = 36 \angle FAO=\alpha =36 , and O A E = 81 α = 45 \angle OAE=81-\alpha =45 . We then have O F = O A × sin 36 \overline { OF } =\overline { OA } \times \sin { 36 } , and O E = O A × sin 45 \overline { OE } =\overline { OA } \times \sin { 45 } , so O E O F = sin 45 sin 36 = 2 2 10 2 5 4 = 2 5 5 \frac { OE }{ OF } =\frac { \sin { 45 } }{ \sin { 36 } } =\frac { \frac { \sqrt { 2 } }{ 2 } }{ \frac { \sqrt { 10-2\sqrt { 5 } } }{ 4 } } =\frac { 2 }{ \sqrt { 5-\sqrt { 5 } } } .

1500 2 + 5 = 214 \left\lfloor \frac { 1500 }{ 2+5 } \right\rfloor =\boxed { 214 }

Nihar Mahajan
Sep 2, 2015

Method 1:

Let H H be orthocenter and H L A B , H K A C HL \perp AB \ , \ HK \perp AC . Note that H H and O O are the isogonal conjugates of each other. By one of the properties of isogonal conjugates ,

\begin{aligned}{\dfrac{OE}{OF} = \dfrac{HL}{HK} \\ = \dfrac{2R\cos A \cos B}{2R\cos A \cos C} \\ = \dfrac{\cos B}{\cos C} \\ = \dfrac{\cos 45^\circ}{\cos 54^\circ} \\ = \dfrac{2}{\sqrt{5-\sqrt{5}}}}\end{aligned} \\ \Rightarrow a+b=2+5=7 \Rightarrow \left \lfloor \dfrac{1500}{7} \right \rfloor = \lfloor 214.28 \rfloor = \boxed{214}

Method 2:

We know that O E = 1 2 ( B H ) = 2 R cos B 2 , O F = 1 2 ( C H ) = 2 R cos C 2 OE = \dfrac{1}{2}(BH) = \dfrac{2R\cos B}{2} \ , \ OF = \dfrac{1}{2}(CH) = \dfrac{2R \cos C}{2}

Clearly , it follows that O E O F = cos B cos C \dfrac{OE}{OF} = \dfrac{\cos B}{\cos C} and compute the required quantities same as in method 1 1 . :)

It is not true that O E / O F = H L / H K OE/OF = HL/HK ; since you are dropping the perpendiculars from H H , you have to do the same with O O . If P P is the foot of the perpendicular from O O to A C AC , and Q Q is the foot of the perpendicular from O O to A B AB , then O P / O Q = H L / H K OP/OQ = HL/HK .

Jon Haussmann - 5 years, 9 months ago

Log in to reply

Sorry , my misconception. I read the page of Isogonal Conjugates from Wolfram Mathworld which did not mention the data properly. Thanks , let me edit the question :)

Nihar Mahajan - 5 years, 9 months ago

Did you construct H H for the fun of it? because it is really not necessary lol

btw as you have experienced, online resources even the likes of Wolfram may contain incorrect information. For this "property" you learned, you should have tried to prove it yourself first. The topic of Isogonal Conjugates is very interesting and still considered modern and fresh. It is very probable that one could discover some unknown properties involving its configuration(It happened to me).

Xuming Liang - 5 years, 9 months ago

Log in to reply

Haha , I think I did construct it for fun xD ;P .I have added method 2 also.

I have already proved this property of perpendiculars (Using cyclicity and similarity). Yeah , I got mislead due to inadequate information on Wolfram Mathworld.

Nihar Mahajan - 5 years, 9 months ago

What are isogonal conjugates?? @Nihar Mahajan

Harsh Shrivastava - 5 years, 9 months ago

Log in to reply

In Δ A B C \Delta ABC , cevians A P 1 a n d A P 2 \overrightarrow{AP_1} \ and \ \overrightarrow{AP_2} are said to be isogonal conjugates of each other if and only if bisector of A \angle A also bisects angle between them.

Nihar Mahajan - 5 years, 9 months ago

Log in to reply

Oh! Thanks!

Btw how's NTSE prep goin??

Harsh Shrivastava - 5 years, 9 months ago

Log in to reply

@Harsh Shrivastava Actually I am not much concentrating on NTSE bcoz its much based on school sylabi. I am concentrating on RMO , JEE , NSEA.

Nihar Mahajan - 5 years, 9 months ago

@Harsh Shrivastava How's your preparation going on?

Nihar Mahajan - 5 years, 9 months ago

Log in to reply

@Nihar Mahajan I am also not focussing on NTSE... :P

Harsh Shrivastava - 5 years, 9 months ago

N o t e t h a t A F = F B , A E = E C , A O = R = A F S i n 54 , S i n 54 = 1 + 5 4 , S i n 45 = 1 2 , b y S i n L a w A E = 2 1 + 5 4 A F = 4 2 1 + 5 A F A E 2 = 4 3 + 5 A F 2 Note ~that ~AF=FB,~~AE=EC,~~AO=R=\dfrac{AF}{Sin54},~~\\ Sin54=\dfrac{1+\sqrt5} 4,~~Sin45=\dfrac 1 {\sqrt2},\\ by~Sin~Law~AE=\dfrac{\sqrt2}{\frac{1+\sqrt5} 4}*AF=\dfrac{4\sqrt2}{1+\sqrt5}*AF\\ \therefore~ AE^2=\dfrac 4 {3+\sqrt5}*AF^2\\ A O 2 = 8 3 + 5 A F 2 . O E O F = A O 2 A E 2 A O 2 A F 2 = { 8 3 + 5 4 3 + 5 8 3 + 5 1 } = { 8 4 8 ( 3 + 5 ) } = 2 5 5 = a b b . 1500 2 + 5 = 214 AO^2=\dfrac 8 {3+\sqrt5}*AF^2. \\ \therefore \dfrac{OE}{OF}= \sqrt{ \dfrac { AO^2 - AE^2 } {AO^2-AF^2} }\\ =\sqrt{ \left \{ \dfrac{ \frac 8 {3+\sqrt5 } -\frac 4 {3+\sqrt5} } { \dfrac 8 {3+\sqrt5 }- 1 } \right\} }\\ =\sqrt{\left \{\dfrac { 8 - 4}{ 8 - (3+\sqrt5 ) } \right\} } \\ =\dfrac 2 {\sqrt{5 - \sqrt5}}=\dfrac a {\sqrt{b-\sqrt b}}.\\ \therefore \left \lfloor \dfrac{1500}{2+5} \right \rfloor \\ =\Large \color{#D61F06}{214}

Pulkit Deshmukh
Sep 3, 2015

Hey @Nihar Mahajan

Which standard you are actually in,Are you 14 years old???

Yes I am 14 years old. I am studying in Std. 10. What about you?

Advice: Please refrain from posting comments as a solution. If you want to comment/ask something , you can click the "Reply button" at the bottom of solution section and type your comment/opinion in the box saying "Write a comment or ask a question...".Thanks.

Nihar Mahajan - 5 years, 9 months ago

Log in to reply

So awesome in geometry at the age of 14;You are a Genius!

Pulkit Deshmukh - 5 years, 9 months ago

Log in to reply

Thank you very much! :)

Nihar Mahajan - 5 years, 9 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...