What is the identity matrix of a #2xx2# matrix?
The identity matrix of a 2x2 matrix is:
To find the identity matrix of an nxn matrix you simply put 1's for the main diagonal (from the top left to the bottom right http://en.wikipedia.org/wiki/Main_diagonal) of the matrix, and zeroes everywhere else (so in the "triangles" below and above the diagonals). In this case it doesn't really look like a triangle but for bigger matrices there is the appearance of a triangle above and below the main diagonal. The link shows a visual representation of the diagonals.
Also, for an nxn matrix, the number of ones in the main diagonal actually equals the number of n. In this case, it's a 2x2 matrix, n=2, so there are 2 ones in the diagonal. In a 5x5 matrix there will be 5 ones in the diagonal.