Is it possible to construct a square if only one of its diagonals is given?
Yes
If the length of diagonal of a square is known, we can find out the length of the side of the square. For e.g.
If diagonal length is 'a' in △ABC, using Pythagoras theorem
AB2+BC2=AC2
x2+x2=a2
2x2= a2
x2=a22
x=a√2
Now, we know that square is a regular quadrilateral. So, all the sides are equal to each other and all the angles are equal to each other. We hence have total of 8 measurements (4 sides + 4 angles). Hence, it is possible to construct a square if only one of its diagonals is known.