alleryIf $(d,m,n)$ are direction cosines of a line, which relation must hold?
A$d+m+n=1$
B$d^2+m^2+n^2=1$
C$dmn=1$
D$d^2+m^2+n^2=0$
Answer & Solution
Correct answer: B. $d^2+m^2+n^2=1$
Direction cosines are the cosines of the angles a line makes with the coordinate axes.
**Hence their squares add to 1:** $d^2+m^2+n^2=1$.
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