The Handy Math Answer Book

What is the identity matrix?

The identity matrix is the n-by-nmatrix that has all ones down the main diagonal and
zeroes everywhere else; it must also be a square. The following is the identity matrix.

Or in more formal and explicit terms:

What happens when you multiplya matrixby the identity matrix?

When you multiply any n-by-nmatrix by the identity matrix, you get that same matrix
back again. Therefore, let the letter Irepresent the n-by-nidentity matrix, and Arep-

1

0

0

0

1

0

0

0

1

hh

g

g

j

g

h

R

T

S

S

S

S

S

V

X

W

W

W

W

W

1

0

0

0

1

0

0

0

1

R

T

S

S

SS

V

X

W

W

WW

159

ALGEBRA

All that remains of the once-great Maya civilization are remarkable ruins such as these in the Yucatan Peninsula
of Mexico. They are reminders of an advanced culture that had made great strides in mathematics, including the
use of matrices. The Image Bank/Getty Images.