Converting Linear Equations in Standard Form to Symmetric Form

The general equation or standard equation of a straight line is given by

ax  + by + c = 0


Where a and b are any constants and also either a \ne 0 or b \ne 0.
Putting y = 0 in the above standard equation of a line, we have

\begin{gathered} ax + b\left( 0 \right) + c = 0 \\ \Rightarrow ax =  - c \\ \Rightarrow x =  - \frac{c}{a} \\ \end{gathered}


This shows that the line is passing through the point \left( { - \frac{c}{a},0} \right).
Since we have \tan  \alpha  = m =  - \frac{a}{b}, so by squaring {\tan ^2}\alpha   = {\left( {\frac{a}{b}} \right)^2} \Rightarrow {\tan ^2}\alpha  = \frac{{{a^2}}}{{{b^2}}}

\begin{gathered} \Rightarrow {\sec ^2}\alpha  - 1 = \frac{{{a^2}}}{{{b^2}}} \\ \Rightarrow {\sec ^2}\alpha  = \frac{{{a^2}}}{{{b^2}}} + 1 \\ \Rightarrow {\cos ^2}\alpha  = \frac{{{b^2}}}{{{a^2} + {b^2}}} \\ \Rightarrow \cos \alpha  = \pm \frac{b}{{\sqrt {{a^2} + {b^2}} }} \\ \end{gathered}

\begin{gathered} \Rightarrow {\sin ^2}\alpha  = 1 - {\cos ^2}\alpha \\ \Rightarrow {\sin ^2}\alpha  = 1 - \frac{{{b^2}}}{{{a^2} + {b^2}}} \\ \Rightarrow {\sin ^2}\alpha  = \frac{{{a^2} + {b^2} - {b^2}}}{{{a^2} +    {b^2}}} \\ \Rightarrow {\sin ^2}\alpha  = \frac{{{a^2}}}{{{a^2} + {b^2}}} \\ \Rightarrow \sin \alpha  = \pm \frac{a}{{\sqrt {{a^2} + {b^2}} }} \\ \end{gathered}

Since equation of straight line in symmetric form is

\frac{{x  - {x_1}}}{{\cos \alpha }} = \frac{{y - {y_1}}}{{\sin \alpha }}


Now putting {x_1}  =  - \frac{c}{a},\,\,{y_1} = 0,\,\,\cos  \alpha  =   \pm \frac{b}{{\sqrt {{a^2} + {b^2}} }},\,\,\sin \alpha  =  \pm  \frac{a}{{\sqrt {{a^2} + {b^2}} }} in the standard equation of symmetric form, we have

\begin{gathered} \frac{{x - \left( { - \frac{c}{a}}  \right)}}{{ \pm \frac{b}{{\sqrt {{a^2} + {b^2}} }}}} = \frac{{y - 0}}{{ \pm  \frac{a}{{\sqrt {{a^2} + {b^2}} }}}} \\ \Rightarrow \frac{{x + \frac{c}{a}}}{{ \pm  \frac{b}{{\sqrt {{a^2} + {b^2}} }}}} = \frac{y}{{ \pm \frac{a}{{\sqrt {{a^2} +  {b^2}} }}}} \\ \end{gathered}


This is the equation of line in symmetric form transformed from its general form or standard form of straight line.

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