>>10870947Because when you draw a horizontal line near the bottom through the vertex of alpha, you get another equilateral triangle. So the side lengths of this slightly smaller triangle are the same as the width of the square. If you really need me to spell it out for you, that means that the height of the square is the same as the length of each side. So the triangle formed by alpha, the altitude of the square, and one of these sides is isoceles. Now the top angle is 30 degrees, and the bottom two angles are equal; 75.
