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Consider the integer powers of 7: a(n) = 7^n:
7, 49, 343, 2401, 16807,... 117649, ...

Prove that the second from right digit in all those numbers (excluding the 1st) is either 0 or 4.

(No Solution Yet, 1 Comments) Submitted on 2018-12-16 by Ady TZIDON    

Three concentric circles each have radii 8, 15 and 17.

Find the largest possible area of the equilateral triangle with one vertex on each circle.
(No Solution Yet, 2 Comments) Submitted on 2018-12-15 by Danish Ahmed Khan    

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