Distance between A and B 
= Distance between C and D 
= sqrt((4 - 1)^2 + (5 - 2)^2) 
= sqrt(3^2 + 3^2) 
= sqrt(2 * 3^2) 
= sqrt(3^2) * sqrt(2) 
= 3sqrt(2) 
 
Distance between B and C 
= Distance between A and D 
= sqrt((4 - 3)^2 + (5 - 0)^2) 
= sqrt(1^2 + 5^2) 
= sqrt(26) 
 
Since sqrt(26) is more than 3sqrt(2), the length must be sqrt(26).
Hope this helps you.