Question

Which identity must be satisfied by the direction cosines \[(l,m,n)\] of a line?

Options

• \[l+m+n=1\]

• \[l^2+m^2+n^2=1\]

• \[l^2+m^2+n^2=0\]

• \[lmn=1\]

Answer 1

For any set of direction cosines of a line, the sum of their squares is one. Thus, \[l^2+m^2+n^2=1\] is the main identity for direction cosines.