The Greatest of the Rejected Ones

Geometry Level 2

In Δ A B C \Delta ABC

  • A B = A C AB = AC ;
  • P P is a point between B B and C C and
  • P B = 3 , P C = 11 PB=3, PC=11 .

Find the largest number that cannot be the length of P A PA .


Bonus : Generalize this for general length of B C BC and general location of P P between B B and C C .


The answer is 4.

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1 solution

William Crabbe
Apr 20, 2018

P B = 3 , P C = 11 PB=3, PC=11 P B + P C = 14 PB+PC=14 By definition of an isosceles triangle, A A must be located above the midpoint of B C BC ; if D D were the midpoint, B D BD would be of length P B 2 = 14 2 = 7 \frac{PB}{2}=\frac{14}{2}=7 . Remember that A A is above the midpoint. Because a line segment with 3 points is not a triangle, we can use the lengths of B D BD and P B PB to find our answer: B D P B = 7 3 = 4 BD-PB=7-3=4 Final answer: P A > 4 PA>4

Bonus: P A > a b s ( B C 2 P B ) PA\gt abs\left(\frac{BC}{2}-PB\right)

Yeah! That is it!

Muhammad Rasel Parvej - 3 years, 1 month ago

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