The direction cosines of a line are $l$, $m$, $n$. They satisfy:

Question

Accepted Answer

A. $l^2 + m^2 + n^2 = 1$

Direction cosines are the cosines of the angles that a line makes with the three coordinate axes: $l = \cos\alpha$, $m = \cos\beta$, $n = \cos\gamma$.
Key identity: $l^2 + m^2 + n^2 = \cos^2\alpha + \cos^2\beta + \cos^2\gamma = 1$.
This just says the unit vector along the line has unit length when projected onto the three axes.

Answer

$l \cdot m \cdot n = 1$

Answer

$l + m + n = 1$

Answer

$l^2 + m^2 + n^2 = 0$

The direction cosines of a line are $l$, $m$, $n$. They satisfy:

Keep practising Unit 8B: Volumes of Revolution and Cross-Sections