# Let ABCD be a convex quadrilateral with AB = a, BC = b, CD = c and DA = d. Suppose $a^2 + b^2 + c^2 + d^2 = ab + bc + cd + da$ and the area of ABCD is 60 square units. If the length of one of the diagonals is 30 unit, determine the length of the other diagonal.

Saurabh Koranglekar