Question

In the following cases, determine whether the given planes are parallel or perpendicular, and in case they are neither, find the angles between them.
$2x+y+3z-2=0$ and $x-2y+5=0$.

Answer 1

The direction ratios of normal to the plane, $L_1:a_{1}x+b_{1}y+c_{1}z=0$ are $a_1,b_1,c_1$ and $L_2:a_{1}x+b_{2}y+c_{2}z=0$ are $a_2,b_2,c_2$
$L_1\parallel L_2$, if $\frac{a_1}{a_2}=\frac{b_1}{b_2}=\frac{c_1}{c_2}$
$L_1\perp L_2$, if $a_1{a}_2+b_1{b}_2+c_1{c}_2=0$
The angle between $L_1$ and $L_2$ is given by,
$\theta ={\cos }^{-1}\left|\frac{a_1{a}_2+b_1{b}_2+c_1{c}_2}{\sqrt{a_1^2+b_1^2+c_1^2.\sqrt{a_2^2+b_2^2+c_1^2}}}\right|$
Here the equations of the planse are $2x+y+3z-2=0$ and $x-2y+5=0$
$\Rightarrow a_1=2,b_1=-2,c_1=3$ and $a_2=1,b_2=-2,c_2=0$
Now $a_1{a}_2+b_1{b}_2+c_1{c}_2=2\times 1+1\times (-2)+3\times 0=0$
Thus, the given planes are perpendicular to each other.