Chapter Ten — Applications of Matrix Algebra
The Humongous Book of Algebra Problems
209
Note: Problems 10.3–10.5 refer to matrix A defined below.
10.3 Construct matrix B that serves as the additive identity for matrix A and verify
that A + B = A.
As explained in Problem 1.36, the additive identity for real numbers is 0.
Adding 0 to any real number x does not change the value of x.
x + 0 = 0 + x = x
To add matrix B to matrix A without changing the elements of A, every element
of B must be 0. Therefore, the additive identity for any m × n matrix is the zero
matrix 0
m × n
.
Verify that B is the additive inverse by demonstrating that A + B = A.
Note: Problems 10.3–10.5 refer to matrix A defined below.
10.4 Construct matrix C that serves as the multiplicative identity for matrix A.
The identity matrix I
n
is a square matrix with n rows and n columns that
contains 0s for each of its elements except for those along the diagonal that
begins with the element in the first row and the first column.
In this problem, matrix A has two columns, so for the product A · C to exist, C
must have two rows. Therefore, the correct identity matrix is C = I
2
.
A zero
matrix is
exactly what
you’d imagine:
a matrix that
contains nothing but
zeros. It has to have
the same dimensions
of A because
you can’t add
matrices unless
they’re the
same size.
An identity
matrix has ones
along the diagonal
that stretches from the
top-left to the bottom-
right corner. All the
other elements in
the matrix are
zeros.