Question

The position vectors of P and Q are $5\mathbf{i} + 4\mathbf{j} + a\mathbf{k}$ and $- \mathbf{i} + 2\mathbf{j} - 2\mathbf{k}$ respectively. If the distance between them is 7, then the value of a will be

• A. – 5, 1
• B. 5, 1
• C. 0, 5
• D. 1, 0

Answer 1

Answer: (A)

– 5, 1

Answer 2

$7 = \sqrt{(5 + 1)^{2} + (4 - 2)^{2} + (a + 2)^{2}} \Rightarrow a + 2 = \pm 3$

or $a = - 5,1.$