Question

in the square why the diagonals bisect at 90°

Answer 1

Becoz they are the fixed properties .......  Hope it helps u.......

Answer 2

The diagonals of a square bisect each other and meet at 90° The diagonalsof a square bisect its angles. Opposite sides of a square are both parallel and equal in length. All four angles of asquare are equal.
square is a special case of a isosceles trapezoid, kite, parallelogram, quadrilateral, rectangle, rhombus, and trapezoid. The diagonals of a square bisect one another and are perpendicular (illustrated in red in the figure above). In addition, they bisect each pair of opposite angles (illustrated in blue).