1 0 0 0 1 0 0 0 1
For any dimension:
IT = I
Another useful property is the following. Actually, this is really just an application of the definition of I.
A I B = A B
Another easy application of the definition:
I I = I, or I2 = I
These are all obvious; you should not have to memorize anything here. But it does help to look them over.
Compute the following:
0.5 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.5 2 4 6 0 -12 -8 4 0 4=
Hint: you don't actually need to multiply matrices.