A challenging problem from Geometry

 

The triangle ABC is given. Let T be circle passing through vertex A and intersecting sides AB and AC in points D and E respectively. Let T' be circle passing through vertex B and point D, intersecting side BC and the circle T in points F and T, respectively. Prove that the quadrilateral CEFT is cyclic.

For solution click here: Solution

Sum of squares of 5 consecutive natural numbers!

Here's a difficult problem of proving that a given expression is not a perfect square. A direct approach will be a nightmare ( even not sure it can be proved) but use of a simple property of perfect squares will ease the problem. We know that any perfect squares leaves a remainder either 1 or 0 when being divided by 3 or 4. This simple result would be used to solve the problem.