The maximum number of different elements required to form a symmetric matrix of order $6$ is

$$\begin{gathered} We are given that the order of a symmetric matrix is 6 \\ as we know the maximum elements required to form a symmetric \\ matrix of order \text{'}n\text{'} is given by \\ maximum number of elements=\frac{n\left(n+1\right)}{2} \\ Here n=6 \\ Thus \frac{6\left(6+1\right)}{2}=3\times 7=21 \end{gathered}$$