Let A , B , C , D be points on a line in that order and A C = 1 1 , B C = 5 , B D = 6 0 . The circles with diameters A C , B D intersect at E , F . Let P be a point on the circle with diameter A D such that P , F are on the same side of A D and A P = D P . Find P E
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Please fix the latex to make your solution more presentable. Good observation on the harmonic division!
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Thank you! It was my first time using LaTeX, so I did not notice it. Now, it looks so much better!
Using geometry the solution is as under.
O 1 , O 2 , O 3 are the centers of circles on diameters 11, 60 and 66. G is the point of intersection of EF and AD. Applying Cos Rule to △ O 1 O 2 E , C o s O 1 O 2 E = 2 ∗ 3 0 . 5 ∗ 3 0 ( 3 6 − 5 . 5 ) 2 + 3 0 2 − 5 . 5 2 = . 9 8 3 6 ∴ S i n O 1 O 2 E = . 0 . 1 8 0 3 I n r t . ∠ e d Δ E G O 2 , E G = 3 0 ∗ S i n O 1 O 2 E = 5 . 4 . G O 2 = 3 0 ∗ C o s O 1 O 2 E . = 2 9 . 5 1 . P E = G O 3 2 + ( P O 3 + G E ) 2 = ( 2 9 . 5 1 − 3 ) 2 + 3 8 . 4 2 = 4 6 . 6 6 9 .
Please enlarge the sketch.If you let A:(0,0), B:(6,0),C:(11,0) & D: (66,0) then we may obtain E as intersection of (x-11/2)²+y²=(11/2)² and (x-36)²+y²=(30)² which turns out to be (396/61,330/61) while P is (33,-33). From which we can determine PE as 33√2 or ~ 46.669
Aah ! It hurts. Just a minor calculation mistake .:(
Using the analytical approach for this problem is very natural. Great job.
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We call O the center of the circle with diameter A D and G the point that satisfies that ∠ D A E = ∠ E A G , ∠ G D E = ∠ E D A and G is on the same side of A D as E . Since we know that A P = P D , then P is the midpoint of the semiarc A D ⌢ . Similarly, P O ⊥ A D , and A O = P O = D O , which is the radius of the circle with diameter A D . We know that B is the harmonic conjugate of D with respect to A and C , because B C A B = 5 6 = 5 5 6 6 = B D A D . We also know that B E ⊥ E D since B D is diameter, then E B is the angle bisector of ∠ A E C , so ∠ A E B = ∠ B E C = 4 5 ° , because A E ⊥ E C since A C is diameter. Using the circle with diameter A C , we notice that ∠ C A E = ∠ C F E and using the one with diameter B D , we notice that ∠ E D B = ∠ E F B . Since ∠ C F E + ∠ E F B = ∠ C F B = ∠ B E C = 4 5 ° , we know that ∠ C A E + ∠ E D B = 4 5 ° . Since we defined ∠ D A E = ∠ E A G and ∠ G D E = ∠ E D A , then ∠ C A G + ∠ G D B = 2 ∗ ( ∠ C A E + ∠ E D B ) = 2 ∗ 4 5 ° = 9 0 ° . Then, ∠ A G D = 9 0 ° , so G is on the circumference of diameter A D . Since E is the incenter of △ A G D , then G E is the angle bisector of ∠ A G D , but since P is the midpoint of the semiarc A D ⌢ , then G P is also the angle bisector of ∠ A G D , so G , E and P are collinear. We know that ∠ P A O = 4 5 ° , since A O = P O and ∠ A O P = 9 0 ° , then ∠ A E P = ∠ E A G + ∠ A G E = ∠ E A G + 4 5 ° = ∠ D A E + 4 5 ° = ∠ D A E + ∠ P A D = ∠ P A E . Then, P A = P E = P D , so P E = P A = A O 2 + P O 2 = 3 3 2 + 3 3 2 = 4 6 . 6 6 9 .