Question

The vectors $3\mathbf{i} + \mathbf{j} - 5\mathbf{k}$ and $a\mathbf{i} + b\mathbf{j} - 15\mathbf{k}$are collinear, if

• A. $a = 3,b = 1$
• B. $a = 9,b = 1$
• C. $a = 3,b = 3$
• D. $a = 9,b = 3$

Answer 1

Answer: (D)

$a = 9,b = 3$

Answer 2

$\frac{3}{a} = \frac{1}{b} = \frac{- 5}{- 15} \Rightarrow a = 9,b = 3.$