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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
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The correct answer is:
$-5,1$
$\begin{aligned} & 7=\sqrt{(5+1)^2+(4-2)^2+(a+2)^2} \Rightarrow a+2= \pm 3 \\ & \text { or } a=-5,1 .\end{aligned}$
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