Prove that the line of section of the planes 5x + 2y – 4z + 2 = 0 and 2x + 8y + 2z – 1 = 0 parallel to the plane 4x – 2y – 5z – 2 = 0.

Let a1,b1 and c1 be the direction ratios of the line 5x + 2y – 4z + 2 = 0 and 2x + 8y + 2z – 1 = 0.

As we know that if two planes are perpendicular with direction ratios as a1, b1 and c1 and a2 , b2 and c2 then


a1a2 + b1b2 + c1c2 = 0


Since, line lies in both the planes, so it is perpendicular to both planes


5a1 + 2b1 – 4c1 = 0 ……(1)


2a1 + 8b1 + 2c1 = 0 ……(2)


Solving equation (1) and (2) by cross multiplication we have,






a = 2k, b = – k and c = 2k


We know that line is parallel to plane a2x + b2y + c2z + d2 = 0 if a1a2 + b1b2 + c1c2 = 0 ……(3)


Here, line with direction ratios a1,b1 and c1 is parallel to plane,


4x – 2y – 5z = 5


So,


2×4 + (– 1)× – 2 + 2× – 5 = 0


8 + 2 – 10 = 0


Therefore, the line of section is parallel to the plane.


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