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When we are actually finding the eqn of a plane parallel to the x axis or any other axis then why do we make the direction ratio of respective axis to be zero ? I mean for example, you represent for a plane parallel to x axis as by+cz=d where a=0 but why?

satyatech
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  • In simple terms: if $(x,y,z)$ belongs to the plane, so does $(x',y,z)$, for any $x'$. This translates to the fact that $x$ cannot appear in the plane equation. –  Jun 08 '15 at 16:20

1 Answers1

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The coefficients $a,b,c$ in the equation of your plane can be interpreted as the direction cosines of the normal vector to the plane.

Direction cosines are a slightly more general version of the idea of "slope" in 2 variables. See http://en.wikipedia.org/wiki/Direction_cosine

By virtue of the normal being perpendicular to the required plane, the corresponding direction coefficient vanishes.

OnceUponACrinoid
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  • i understood what you are saying as cos(90) is zero that's why the normal's direction cosine with x axis is 0 and that's why the term vanishes!! – satyatech Jun 08 '15 at 16:42