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NDA2012MathematicsThree Dimensional GeometryActual

What is the cosine of angle between the planes x+y+z+1=0 and 2 x-2 y+2 z+1=0 ?

Options

  1. A1/2
  2. B1/3
  3. C2/3
  4. DNone of the above

Correct answer

B. 1/3

Step-by-step solution

= 1(2)+1(-2)+1(2) 1^2+1^2+1^2 2^2+(-2)^2+2^2 = 2 3 2 3 = 1 3 .

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