Points A and B lie on a circle Γ with radius 123, such that their tangents intersect at C and satisfy ∠ A C B = 9 0 ∘ . D is a point chosen on minor arc A B of circle Γ . The tangent at D intersect A C and B C at E and F respectively. What is the perimeter of triangle C E F ?
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Great approach!
How do we generalize this problem if we are not given that ∠ A C B = 9 0 ∘ ? What would the perimeter of C E F depend on?
You can generalize the problem by:
C E F = 2 ⋅ tan 2 ∠ A C B r
Where r is the radius of the Γ .
So using this you will need to know the value of ∠ A C B or ∠ A O B .
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Well , how do we derive
C E F = tan 2 ∠ A C B 2 r ?
Please explain . Thank you
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If we draw the line C O , then we get triangle B O C with angle ∠ C B O = 9 0 ∘ . Therefore, using a d j a c e n t = t a n g e n t o p p o s i t e , we can see that:
B C = tan O C B r = tan 2 A C B r
We also know that A C = B C because tangents drawn from a single point are equal in length. Using the method above, we can see that the perimeter is:
tan 2 A C B r − A E + tan 2 A C B r − B F + A E + B F = tan 2 A C B 2 ⋅ r
nice one
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
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i did not get u....please xplain it more elaborately
sip sip
We can think the point D is on the point A or B. So the perimeter will be just double as third side =0
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
Since C is the intersection of the tangents, A B = A C . Further if O is the center of the circle then A O = O B = 1 2 3 Since A C ⊥ B C and A O ⊥ A C ⇒ A O ∥ B C . One can also show that in fact A O = B O = A C = C B
Next the point E lying on A C is the point of intersection of the tangents A E and E D . Thus D E = A E . And similarly D F = F B
Next the P e r i m e t e r ( C E F ) = C E + E F + C F , where E F = D E + D F = A E + B F
⇒ 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 = 2 4 6
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D E = A E and D F = B F , because tangents drawn from a single point are equal in length. Therefore, we can see that E F = A E + B F .
A O B C forms a square, with O being the centre of the circle Γ , so C B = A C = 1 2 3 .
We can see that:
C E = 1 2 3 − A E
C F = 1 2 3 − B F
Therefore the perimeter of C E F is:
C E + E F + C F = 1 2 3 − A E + 1 2 3 − B F + A E + B F = 2 4 6