Answer
a. False.
b. True.
c. False.
d. False.
e. True.
Work Step by Step
a. The addition signs should not be present, per the definition on page 97: $AB=\begin{bmatrix}A\vec{b}_{1}&A\vec{b}_{2}&A\vec{b}_{3}\end{bmatrix}$.
b. This result is labeled formula (2) on page 99.
c. According to Theorem 2(a), matrix multiplication is associative, but as noted in the warning box on page 100, matrix multiplication is generally not commutative.
d. Theorem 3(d) states that the order of multiplication must be reversed: $(AB)^{T}=B^{T}A^{T}$.
e. This is a verbal statement of Theorem 3(b).
