A probability problem by Paola Ramírez

How many is isosceles triangles can be formed in the following grid of 11 × 11 11\times11 with their unequal base on the pink lines?


Thanks to Abbhay Tiwari for his contribution to this problem.


The answer is 5500.

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1 solution

Paola Ramírez
May 6, 2016

First, select the points of the base for this we will look at one horizontal line of points. The two points of the base have to have an odd number of points between them, so the points of the base have to have the same parity. we can selected of ( 5 2 ) \binom{5}{2} (for even points) + ( 6 1 ) +\binom{6}{1} (for odd point), this is equal to 25 25 ways of select the base of one line. Then each base could have 10 10 heights(this is the third point of the triangle). Last, this happens to each pink, there is 20 20 pink line \Rightarrow we get 25 × 10 × 22 = 5500 25\times10\times22=5500

Yep solved it the same way :) May I request you to change the bame of my friend to Abhay Tiwari.

Ashish Menon - 5 years, 1 month ago

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