Let a 1 x + b 1 y + c 1 z + d 1 = 0 and a 2 x + b 2 y + c 2 z + d 2 = 0 be two planes…

Mathematics · JEE Advanced · NTA ExamsThree Dimensional Geometry


Let a1x + b1y + c1z + d1 = 0 and a2x + b2y + c2z + d2 = 0 be two planes, where d1, d2 > 0 . Then origin lies in acute angle if a1a2 + b1b2 + c1c2 < 0 and origin lies in obtuse angle if a1a2 + b1 b2 + c1c2 > 0 .
Further point (x1, y1, z1) and origin both lie either in acute angle or in obtuse angle, if (a1x1 + b1y1 + c1z1 + d1)
(a2x1 + b2y1 + c2z1 + d2) > 0 . One of (x1, y1, z1) and origin lie in acute angle and the other in obtuse angle, if
(a1x1 + b1y1 + c1z1 + d1) (a2x1 + b2y1 + c2z1 + d2) < 0
Given the planes x + 2y – 3z + 5 = 0 and
2x + y + 3z + 1 =0 . If a point P is (2, –1, 2), then
  1. O and P both lie in acute angle between the planes
  2. O and P both lie in obtuse angle
  3. O lies in acute angle, P lies in obtuse angle
  4. O lies in obtuse angle, P lies an acute angle.

Answer

(C) O lies in acute angle, P lies in obtuse angle

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