Question
Find the vector equations of planes which pass through A(1, 2, 3), B (3, 2, 1) and make equal intercepts on the co-ordinates axes. Question is modified Find the cartesian equations of the planes which pass through A(1, 2, 3), B(3, 2, 1) and make equal intercepts on the coordinate axes.

Answer

Case 1 : Let all the intercepts be 0. Then the plane passes through the origin. Then the cartesian equation of the plane is ax + by + cz = 0 …..(1) (1, 2, 3) and (3, 2, 1) lie on the plane. ∴ a + 2b + 3c = 0 and 3a + 2b + c = 0

$\therefore \frac{a}{\left|\begin{array}{ll}2 & 3 \\ 2 & 1\end{array}\right|}=\frac{b}{\left|\begin{array}{ll}3 & 1 \\ 1 & 3\end{array}\right|}=\frac{c}{\left|\begin{array}{ll}1 & 2 \\ 3 & 2\end{array}\right|}$

$\therefore \frac{a}{-4}=\frac{b}{8}=\frac{c}{-4}$

i.e. $\frac{a}{1}=\frac{b}{-2}=\frac{c}{1}$

∴ a, b, c are proportional to 1, -2, 1 ∴ from (1), the required cartesian equation is x – 2y + z = 0. Case 2 : Let the plane make non zero intercept p on each axis.

then its equation is $\frac{x}{p}+\frac{y}{p}+\frac{z}{p}=1$

i.e. x + y + z = p …(2) Since this plane pass through (1, 2, 3) and (3, 2, 1) ∴ 1 + 2 + 3 = p and 3 + 2 + 1 = p ∴ p = 6 ∴ from (2), the required cartesian equation is x + y + z = 6 Hence, the cartesian equations of required planes are x + y + z = 6 and x – 2y + z = 0.

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free