SAT Deja vu

Geometry Level 5

Let A , B , C , D A,B,C,D be points on a line in that order and A C = 11 , B C = 5 , B D = 60 AC=11, BC=5, BD=60 . The circles with diameters A C , B D AC,BD intersect at E , F E,F . Let P P be a point on the circle with diameter A D AD such that P , F P,F are on the same side of A D AD and A P = D P AP=DP . Find P E PE


The answer is 46.6690.

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2 solutions

Raúl Hernández
Oct 9, 2015

We call O O the center of the circle with diameter A D AD and G G the point that satisfies that D A E = E A G \angle DAE = \angle EAG , G D E = E D A \angle GDE = \angle EDA and G G is on the same side of A D AD as E E . Since we know that A P = P D AP = PD , then P P is the midpoint of the semiarc A D \overset{\frown}{AD} . Similarly, P O A D PO\perp AD , and A O = P O = D O AO = PO = DO , which is the radius of the circle with diameter A D AD . We know that B B is the harmonic conjugate of D D with respect to A A and C C , because A B B C = 6 5 = 66 55 = A D B D \frac {AB}{BC} = \frac {6}{5} = \frac {66}{55} = \frac {AD}{BD} . We also know that B E E D BE\perp ED since B D BD is diameter, then E B EB is the angle bisector of A E C \angle AEC , so A E B = B E C = 45 ° \angle AEB = \angle BEC = 45° , because A E E C AE\perp EC since A C AC is diameter. Using the circle with diameter A C AC , we notice that C A E = C F E \angle CAE = \angle CFE and using the one with diameter B D BD , we notice that E D B = E F B \angle EDB = \angle EFB . Since C F E + E F B = C F B = B E C = 45 ° \angle CFE + \angle EFB = \angle CFB = \angle BEC = 45° , we know that C A E + E D B = 45 ° \angle CAE + \angle EDB = 45° . Since we defined D A E = E A G \angle DAE = \angle EAG and G D E = E D A \angle GDE = \angle EDA , then C A G + G D B = 2 ( C A E + E D B ) = 2 45 ° = 90 ° \angle CAG + \angle GDB = 2 * (\angle CAE + \angle EDB) = 2 * 45° = 90° . Then, A G D = 90 ° \angle AGD = 90° , so G G is on the circumference of diameter A D AD . Since E E is the incenter of A G D \bigtriangleup AGD , then G E GE is the angle bisector of A G D \angle AGD , but since P P is the midpoint of the semiarc A D \overset{\frown}{AD} , then G P GP is also the angle bisector of A G D \angle AGD , so G G , E E and P P are collinear. We know that P A O = 45 ° \angle PAO = 45° , since A O = P O AO = PO and A O P = 90 ° \angle AOP = 90° , then A E P = E A G + A G E = E A G + 45 ° = D A E + 45 ° = D A E + P A D = P A E \angle AEP = \angle EAG + \angle AGE = \angle EAG + 45° = \angle DAE + 45° = \angle DAE + \angle PAD = \angle PAE . Then, P A = P E = P D PA = PE = PD , so P E = P A = A O 2 + P O 2 = 3 3 2 + 3 3 2 = 46.669 PE = PA = \sqrt{AO^2 + PO^2} = \sqrt{33^2 + 33^2} = 46.669 .

Please fix the latex to make your solution more presentable. Good observation on the harmonic division!

Xuming Liang - 5 years, 8 months ago

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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!

Raúl Hernández - 5 years, 8 months ago

Using geometry the solution is as under.

Please enlarge the sketch. 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 = ( 36 5.5 ) 2 + 3 0 2 5. 5 2 2 30.5 30 = . 9836 S i n O 1 O 2 E = . 0.1803 I n r t . e d Δ E G O 2 , E G = 30 S i n O 1 O 2 E = 5.4. G O 2 = 30 C o s O 1 O 2 E . = 29.51 . P E = G O 3 2 + ( P O 3 + G E ) 2 = ( 29.51 3 ) 2 + 38. 4 2 = 46.669. O_1, \ O_2\ ,O_3 \text{ are the centers of circles on diameters 11, 60 and 66.}\\ \text{G is the point of intersection of EF and AD.}\\ \text{Applying Cos Rule to } \triangle\ O_1O_2E, \ \ CosO_1O_2E=\dfrac{(36-5.5)^2+30^2 - 5.5^2}{2*30.5*30}= \color{#3D99F6}{.9836}\\ \therefore\ SinO_1O_2E=\color{#3D99F6}{.0.1803}\\ In \ rt.\ \angle ed\ \Delta\ EGO_2,\ EG=30*SinO_1O_2E=5.4.\\ \color{#EC7300}{GO_2=30*CosO_1O_2E.=29.51}.\\ PE=\sqrt{GO_\color{#D61F06}{3}^2+ (PO_3+GE)^2}=\sqrt{(29.51-3)^2+38.4^2} =46.669.

Niranjan Khanderia - 5 years, 8 months ago
Ajit Athle
Oct 9, 2015

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 .:(

Aakash Khandelwal - 5 years, 8 months ago

Using the analytical approach for this problem is very natural. Great job.

Xuming Liang - 5 years, 8 months ago

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