Circle tangent triangle perimeter

Geometry Level 3

Points A A and B B lie on a circle Γ \Gamma with radius 123, such that their tangents intersect at C C and satisfy A C B = 9 0 \angle ACB = 90^\circ . D D is a point chosen on minor arc A B AB of circle Γ \Gamma . The tangent at D D intersect A C AC and B C BC at E E and F F respectively. What is the perimeter of triangle C E F CEF ?


The answer is 246.

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

Oliver Welsh
Aug 19, 2013

D E = A E DE = AE and D F = B F DF = BF , because tangents drawn from a single point are equal in length. Therefore, we can see that E F = A E + B F EF = AE + BF .

A O B C AOBC forms a square, with O O being the centre of the circle Γ \Gamma , so C B = A C = 123 CB = AC = 123 .

We can see that:

C E = 123 A E CE = 123 - AE

C F = 123 B F CF = 123 - BF

Therefore the perimeter of C E F CEF is:

C E + E F + C F = 123 A E + 123 B F + A E + B F = 246 CE + EF + CF = 123 - AE + 123 - BF + AE + BF = \fbox{246}

Moderator note:

Great approach!

How do we generalize this problem if we are not given that A C B = 9 0 \angle ACB = 90^\circ ? What would the perimeter of C E F CEF depend on?

You can generalize the problem by:

C E F = 2 r tan A C B 2 CEF = 2 \cdot \frac{r}{\tan \frac{\angle ACB}{2}}

Where r r is the radius of the Γ \Gamma .

So using this you will need to know the value of A C B \angle ACB or A O B \angle AOB .

Oliver Welsh - 7 years, 9 months ago

Log in to reply

Well , how do we derive

C E F = 2 r tan A C B 2 CEF = \frac{2r}{ \tan \frac{\angle ACB}{2} } ?

Please explain . Thank you

Priyansh Sangule - 7 years, 9 months ago

Log in to reply

If we draw the line C O CO , then we get triangle B O C BOC with angle C B O = 9 0 \angle CBO = 90^\circ . Therefore, using a d j a c e n t = o p p o s i t e t a n g e n t adjacent = \frac{opposite}{tangent} , we can see that:

B C = r tan O C B = r tan A C B 2 BC = \frac{r}{\tan OCB} = \frac{r}{\tan \frac{ACB}{2}}

We also know that A C = B C AC = BC because tangents drawn from a single point are equal in length. Using the method above, we can see that the perimeter is:

r tan A C B 2 A E + r tan A C B 2 B F + A E + B F = 2 r tan A C B 2 \frac{r}{\tan \frac{ACB}{2}} - AE + \frac{r}{\tan \frac{ACB}{2}} - BF + AE + BF = \frac{2 \cdot r}{\tan \frac{ACB}{2}}

Oliver Welsh - 7 years, 9 months ago

Log in to reply

@Oliver Welsh Now I get it , Thank You !

Priyansh Sangule - 7 years, 9 months ago

nice one

Cal Wells - 7 years, 9 months ago

I drew a square, and since point D doesn't really matter (the placement) we set it the third side equal to zero, thus coming to 246

Sameer L. - 7 years, 9 months ago

Log in to reply

i did not get u....please xplain it more elaborately

Vidyasagar Bodakunta - 7 years, 9 months ago

sip sip

Krisna Attayendra - 7 years, 9 months ago

We can think the point D is on the point A or B. So the perimeter will be just double as third side =0

Mukul Suryawanshi
Aug 19, 2013

well,I considered a special case here thats why i reached the answer quickly.the guy who knows its general solution is welcomed here to write it

Dhruv Sharma
Aug 25, 2013

Since C C is the intersection of the tangents, A B = A C AB = AC . Further if O O is the center of the circle then A O = O B = 123 AO = OB = 123 Since A C B C AC \perp BC and A O A C A O B C AO \perp AC \Rightarrow AO \parallel BC . One can also show that in fact A O = B O = A C = C B AO = BO = AC = CB

Next the point E E lying on A C AC is the point of intersection of the tangents A E AE and E D ED . Thus D E = A E DE = AE . And similarly D F = F B DF = FB

Next the P e r i m e t e r ( C E F ) = C E + E F + C F Perimeter(CEF) = CE + EF + CF , where E F = D E + D F = A E + B F EF = DE +DF = AE + BF

C E = A C A E , C F = C B B F P e r i m e t e r ( C E F ) = A C + B C = 246 \Rightarrow CE = AC - AE, CF = CB - BF \Rightarrow Perimeter(CEF) = AC + BC = 246

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...