If adjacent sides of a parallelogram are equal, then the parallelogram is a rhombus. In a rhombus, the diagonals bisect each other and are perpendicular to each other.
Let the lengths of the diagonals be x and y, with x being the length of the diagonal which is the same as the lengths of the two equal sides.
Then, the rhombus can be divided into 4 congruent right triangles with hypotenuse x and legs x/2 and y/2.
By the Pythagorean Theorem, (x/2)2 + (y/2)2 = x2.
So, (3/4)x2 = (1/4)y2
√3x = y
y/x = √3 (or equivalently, x/y = √3/3)