Squares

A square is both a rectangle and a rhombus: four right angles and four equal sides. Its diagonal cuts it into two right triangles whose legs are both a side of the square.

With side 4: d^2 = 4^2 + 4^2 = 32, so d = √(32) = 4×√(2). Every square's diagonal is its side times √2. The ticks say BC = AB, so pick Marked equal sides for the second leg before Pythagorean theorem.

Triangle ABC is isosceles with a right angle at B, so its other two angles are 45° each: the diagonal cuts each corner in half.