alleryIf $(l,m,n)$ are the direction cosines of a line, then which relation must hold?
A$l+m+n=1$
B$l^2+m^2+n^2=1$
C$l^2-m^2+n^2=1$
D$lm+mn+nl=1$
Answer & Solution
Correct answer: B. $l^2+m^2+n^2=1$
Direction cosines are the cosines of the angles the line makes with the coordinate axes.
**Therefore they satisfy the standard identity $l^2+m^2+n^2=1$.**
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