Invoke Pythagoras' Theorem. The diagonal of the square forms the common hypotenuse of 2 right-angled triangles. Calculate the value of the diagonal squared. Using PT, the result of this will be equal to the sum of the squares of 2 of the sides. Thus. Being a square, each side is of equal length, therefore the square of each side will be half that of the hypotenuse (diagonal). To calculate the length of one side, work out the square root of this derived value.
If D is length of diagonal, and S is length of side of square, the formula is :-
S = Square root of (D squared / 2)
If D is length of diagonal, and S is length of side of square, the formula is :-
S = Square root of (D squared / 2)