# Find the Points on Z-axis Which Are at a Distance √ 21 from the Point (1, 2, 3). - Mathematics

Find the points on z-axis which are at a distance $\sqrt{21}$from the point (1, 2, 3).

#### Solution

Let the point be A (0, 0, z).Then,

AP = $\sqrt{21}$

$\Rightarrow \sqrt{\left( 0 - 1 \right)^2 + \left( 0 - 2 \right)^2 + \left( z - 3 \right)^2} = \sqrt{21}$
$\Rightarrow \left( - 1 \right)^2 + \left( - 2 \right)^2 + \left( z - 3 \right)^2 = 21$
$\Rightarrow 1 + 4 + \left( z - 3 \right)^2 = 21$
$\Rightarrow \left( z - 3 \right)^2 = 21 - 5$
$\Rightarrow \left( z - 3 \right)^2 = 16$
$\Rightarrow z - 3 = \pm 4$
$\Rightarrow z - 3 = 4 or z - 3 = - 4$
$\Rightarrow z = 7 or z = - 1$

Hence, the coordinates of the required point are (0, 0, 7)  and (0, 0, −1).

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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 28 Introduction to three dimensional coordinate geometry
Exercise 28.2 | Q 7 | Page 9