Not your average shuriken

Geometry Level 4

A circle of diameter 14 14 has two identical right triangles that are circumscribed around the two semicircles created by the line A B \overline { AB } cutting the circle in half.

If the short legs of the right triangles are 11 11 as shown, the length of A B \overline { AB } can be written as a b \frac { a }{ b } , where a a and b b are co-prime integers. Find a b ab .

Try another geometry problem: "Star Stumper" .


The answer is 10710.

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.

10 solutions

Otto Bretscher
May 2, 2015

A B = 2 x = 14 cos ( 2 θ ) = 14 1 2 sin 2 θ = 14 1 2 × 7 2 1 1 2 + 7 2 = 595 18 AB=2x=\frac{14}{\cos{(2\theta})}=\frac{14}{1-2\sin^2\theta}=\frac{14}{1-\frac{2\times{7^2}}{11^2+7^2}}=\frac{595}{18}

Moderator note:

Splendid as always!

Wow. Splendid!

Minh Tran - 6 years, 1 month ago

The Sage strikes again! :)

Vincent Miller Moral - 5 years, 10 months ago

Guys!! that two lines are || . Be simple. simply you will get answer.

Lalit Jena - 5 years, 5 months ago
Venture Hi
Apr 22, 2015

solve for x and y.

Just for sake of variety ....

Let O O be the center of the circle and T T be the point of tangency of the "top" right triangle with the circle. Also, let the lower-left vertex of this triangle be Q Q and its uppermost vertex be P . P.

Then Δ P Q B \Delta PQB and Δ O T B \Delta OTB are similar triangles. Now let O B = x |OB| = x and T B = y . |TB| = y. Since P T = P Q = 11 |PT| = |PQ| = 11 and Q O = 7 , |QO| = 7, we have by similarity that

P Q Q B = O T T B 11 7 + x = 7 y 11 y 7 x = 49. \dfrac{|PQ|}{|QB|} = \dfrac{|OT|}{|TB|} \Longrightarrow \dfrac{11}{7 + x} = \dfrac{7}{y} \Longrightarrow 11y - 7x = 49. (i),

and also that

P Q P B = O T O B 11 11 + y = 7 x 11 x 7 y = 77 , \dfrac{|PQ|}{|PB|} = \dfrac{|OT|}{|OB|} \Longrightarrow \dfrac{11}{11 + y} = \dfrac{7}{x} \Longrightarrow 11x - 7y = 77, (ii).

Multiplying (i) by 7 7 and (ii) by 11 11 and adding the resulting equations gives us that

49 x + 121 x = 7 49 + 11 77 x = 1190 72 = 595 36 . -49x + 121x = 7*49 + 11*77 \Longrightarrow x = \dfrac{1190}{72} = \dfrac{595}{36}.

Now A B = 2 x = 595 18 , |AB| = 2x = \dfrac{595}{18}, so a b = 595 18 = 10710 . ab = 595*18 = \boxed{10710}.

Niranjan Khanderia - 3 years ago
Alex Zhong
Apr 21, 2015

Let point C , D C,D be the intersections of line A B \overline{AB} through the circle from left to right. Let point E E be the other vertex of the right triangle on the top semicircle as shown. B E \overline{BE} is the tangent line that intersects with the circle at point F F .

Since both E C \overline{EC} and E F \overline{EF} are tangents to the circle, E C = E F = 11. EC=EF=11. Let B D = x BD = x and B F = y BF=y , by the power of the point theorem, we have

y 2 = x ( x + 14 ) y 2 x 2 = 14 x . y^2=x(x+14) \implies y^2-x^2=14x. (1)

According the Pythagorean Theorem, 1 1 2 + ( x + 14 ) 2 = ( y + 11 ) 2 . 11^2+(x+14)^2 = (y+11)^2.

Regroup the equation and substitute (1) we have 121 + 22 y = 14 x + 317. 121 + 22y = 14x+317.

Therefore, y = 7 ( x + 14 ) 11 . y=\dfrac{7(x+14)}{11}. (2)

Substitute (2) into (1),

121 ( x 2 + 14 x ) = 49 ( x 2 + 28 x + 196 ) , x = 343 36 . 121(x^2+14x)=49(x^2+28x+196), \implies x=\dfrac{343}{36}.

Thus, A B = 2 × 343 36 + 14 = 595 18 = a b . AB= 2\times \dfrac{343}{36} + 14 = \dfrac{595}{18} = \dfrac ab.

We have a b = 595 × 18 = 10710 . ab = 595 \times 18 = \boxed{10710}.

ah.. I too got it with a same approach

Rishabh Tripathi - 6 years, 1 month ago

Since the triangles are similar you can look at the figure like this.

AB = 14 + 2AC. We can use the formula r = Area of triangle (A)/semi-perimeter of triangle (s). Since you're given the radius (7) and the base (22), you can get the value of h. h = 935/36.

Using Pythagorean theorem. You can get the value of the other leg. The value of the other leg is 847/36.

AC = 847/36 - 14

AC = 343/36

AB = 14 + 2(343/36)

AB = 595/18

ab = 595*18 = 10710


A B = 2 ( L o n g l e g ) D i a m e t e r = 2 11 T a n { 2 T a n 1 ( 7 / 11 ) } 14 = 22 2 7 11 1 ( 7 11 ) 2 14 = 847 18 14 = 595 18 = a b . a b = 10710. AB=2*(Long~ leg)~-~Diameter=2*11*Tan\{2*Tan^{-1}(7/11)\}~-~14=22*\dfrac{2*\frac 7{11} }{1-(\frac 7{11})^2}~-~14=\dfrac{847}{18}~-~14=\dfrac{595}{18}=\dfrac a b.\\ \therefore~a*b=10710.

Firstly, B D = y BD=y and B G = x BG=x

Since C E CE and D E DE are tangents, C E = E D = 11 CE=ED=11

C F = F G = 14 2 = 7 CF=FG=\frac{14}{2}=7 since B D BD is a tangent.

Now applying Pythagorean Theorem to the right triangles F D B FDB and E C B ECB , we have the following pair of equations:

7 2 + y 2 = ( 7 + x ) 2 \boxed{7^{2}+y^{2} = (7+x)^{2}} and 1 1 2 + ( 14 + x ) 2 = ( 11 + y ) 2 \boxed{11^{2} + (14+x)^{2} = (11+y)^{2}}

Solving for x x , we come across a quadratic equation 36 x 2 + 161 x 4802 = 0 36x^{2}+161x-4802=0 . The only legitimate value of x x happens to be 343 36 \frac{343}{36} .

Now, we have A B = 14 + 2 x = 595 18 AB=14+2x=\boxed{\frac{595}{18}}

Multiplying their numerator and denominator, we have 595 × 18 = 10710 595 \times 18 = \boxed{10710}

Stephen Gomez
Jun 9, 2016

I hope it helped. :)

William Isoroku
Apr 25, 2015

Connect center of circle O O to a point C C on the point of tangency of the upper right triangle.

Connect these to points to get O C OC , the radius, which is 7 7 .

This forms 2 right triangles that are similar by A A AA .

Call C B = x CB=x

We set up a proportion: h y p h y p = l e g l e g \frac{hyp}{hyp}=\frac{leg}{leg}

The hypotenuse of the smaller right triangle is x 2 + 7 2 \sqrt{x^2+7^2}

Substitute the known values into the proportion: x 2 + 7 2 11 + x = 7 11 \frac{\sqrt{x^2+7^2}}{11+x}=\frac{7}{11}

Solve this equation to get x = 539 36 x=\frac{539}{36}

Thus A B = 2 ( ( 539 36 ) 2 + 7 2 7 ) + 14 AB=2(\sqrt{(\frac{539}{36})^2+7^2}-7)+14 (from the Pythagorean theorem for the smaller triangle)

A B = 595 18 AB=\boxed{\frac{595}{18}}

Josh Banister
Apr 23, 2015

Firstly, notice that C E CE and B E BE are both tangents to the circle. This means that angles B E O \angle BEO and O E C \angle OEC are the same. tan ( O E C ) = 7 / 11 \tan(\angle OEC) = 7/11 so t a n ( B E C ) = tan ( 2 O E C ) = 2 7 11 1 ( 7 11 ) 2 = 77 36 tan(\angle BEC) = \tan(2\angle OEC) \\ = \frac{2\frac{7}{11}}{1 - (\frac{7}{11})^2 } \\ = \frac{77}{36}

From tan ( B E C ) = C B 11 \tan(\angle BEC) = \frac{CB}{11} , C B = 11. 77 36 = 847 36 CB = 11.\frac{77}{36} = \frac{847}{36} As the diagram is symmetrical and D B = C B C D = 847 36 14 = 343 36 DB = CB - CD= \frac{847}{36} - 14 = \frac{343}{36} A B = A C + C D + D B = 343 36 + 14 + 343 36 = 595 18 AB = AC + CD + DB = \frac{343}{36} + 14+ \frac{343}{36} \\ = \boxed{\frac{595}{18}}

Multiplying out gives the answer.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...