What is the maximum number of different elements required to form a symmetric matrix of order 12?

72 74 78 76

78

Matrix of order 12 require 12 × 12 = 144 elements. 12 elements for $i=j$ $\frac{144-12}{2}=66$ elements for $i<j$ ∴ Maximum number of different elements required are $66+12=78$