Question 390575
From the picture you sent, and using the distance formula,
PR = {{{sqrt((a-0)^2 + (0-b)^2) = sqrt(a^2 + b^2)}}}, and 
OQ = {{{sqrt((a-0)^2 + (b-0)^2) = sqrt(a^2 + b^2)}}}.
Therefore PR = OQ, and the two diagonals are congruent.