Question
If the distance between the points (a, 2, 1) and (1, -1, 1) is 5, then a _______.

Answer

If the distance between the points (a, 2, 1) and (1, -1, 1) is 5, then a = 5 or -3.
Solution:
Given points are (a, 2, 1) and (1, -1, 1).
$\therefore\sqrt{(\text{a}-1)^2+(2+1)^2+(1-1)^2}=5$ (Given)
$\Rightarrow(\text{a}-1)^2+9+0=25$
$\Rightarrow\text{a}^2-2\text{a}-15=0$
$\Rightarrow(\text{a}-5)(\text{a}+3)=0$
$\therefore\text{a}=5\ \text{or}\ -3$

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