Question:
If the distance between the points A(4, p) and B(1, 0) is 5, then
(a) p = 4 only
(b) p = −4 only
(c) p = ±4 only
(d) p = 0
Solution:
(c) p = ±4 only
The given points are A(4, p) and B(1, 0) and AB = 5.
Then, $\left(x_{1}=4, y_{1}=p\right)$ and $\left(x_{2}=1, y_{2}=0\right)$
Therefore,
$A B=5$
$\Rightarrow \sqrt{\left(x_{2}-x_{1}\right)^{2}+\left(y_{2}-y_{1}\right)^{2}}=5$
$\Rightarrow \sqrt{(1-4)^{2}+(0-p)^{2}}=5$
$\Rightarrow(-3)^{2}+(-p)^{2}=25$
$\Rightarrow 9+p^{2}=25$
$\Rightarrow p^{2}=16$
$\Rightarrow p=\pm \sqrt{16}$
$\Rightarrow p=\pm 4$