Mystic cevians

Geometry Level 5

Let H H be the orthocentre and O O the circumcentre of a triangle A B C ABC with the length of side B C BC equal to 2014 2014 . Cevians are constructed from vertices B B and C C so as to be tangent to γ \gamma , the circle passing through A A and H H and congruent to the circle centred at O O touching B C BC .

Let the cevians through B B and C C touch the circle γ \gamma at T T and U U respectively. If B T BT and C U CU are of integer lengths, what is their sum?

Aside: On the query of some, this is indeed an original problem of mine!


The answer is 2774.

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1 solution

Aditya Kumar
Dec 26, 2014

It will be recalled that the length A H AH is twice the perpendicular distance of O O from B C BC . This elementary result can be proved by the similiarity of triangles A H G AHG and D O G DOG noting the Euler line and that the median is split in a 2 : 1 2:1 ratio by G G . (Here, G G is the centroid and D D the midpoint of B C BC .)
Suppose K , L , M K,L,M be the feet of altitudes from A , B , C A,B,C respectively. So, the circle γ \gamma has A H AH for diameter. This implies that it is the circle circumscribing the quadrilateral A L H M ALHM , because A H AH subtends a right angle at the point of intersection of γ \gamma and A C AC ( other than A A ) and that makes it coincident with L L . A similiar argument for M M .

Now use the secant theorem as:

B M × B A = B T 2 BM \times BA =BT^2

But by the cyclic quadrilateral M A C K MACK , again the secant theorem yields: B M × B A = B K × B C BM \times BA = BK \times BC

We conclude: B T 2 = B K × B C BT^2 = BK \times BC

By a similiar argument: C U 2 = C K × C B CU^2 = CK \times CB

An intuitively obvious addition step yields: B T 2 + C U 2 = B C 2 = 201 4 2 BT^2 + CU^2 = BC^2 = 2014^2

Now then, B T C U 2014 BT-CU-2014 form a pythagorean triplet with 2014 2014 as the largest member. A simple way to work out the B T BT and C U CU is to factorize 2014 2014 and hunt a triplet with a factor as the largest in the triplet viz. hunting for x y 19 x-y-19 or x y 2 x-y-2 or x y 53 x-y-53 . The latter works : 28 45 53 28-45-53 . Magnify by 38 38 to 1064 1710 2014 1064-1710-2014 .

It so happens that this is the only triplet.(Why?)

The answer : 1064 + 1710 = 2774. 1064+1710 = 2774.

Hope, that was exhaustive. And error free!

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