## Quick Reference

(in 3-dimensional space)

A straight line in 3-dimensional space can be specified as the intersection of two planes. Thus a straight line is given, in general, by two linear equations, *a*_{1}*x*+*b*_{1}*y*+*c*_{1}*z*+*d*_{1}=0 and *a*_{2}*x*+*b*_{2}*y*+*c*_{2}*z*+*d*_{2}=0. (If these equations represent identical or parallel planes, they do not define a straight line.) Often it is more convenient to obtain parametric equations for the line, which can also be written in ‘symmetric form’. See also vector equation (of a line).

*Subjects:*
Mathematics.

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