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Q. 4.7  Given a + b + c + d = 0, which of the following statements are correct:

(a) a, b, c, and d must each be a null vector,

(b) The magnitude of (a + c) equals the magnitude of ( b + d),

(c) The magnitude of a can never be greater than the sum of the magnitudes of b, c, and d,

(d) b + c must lie in the plane of a and d if a and d are not collinear, and in the line of a and d, if they are collinear?

Answers (1)

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(a)Incorrect: The sum of three vectors in a plane can be zero. So it is not a necessary condition that all of a,b,c,d should be null vector.

(b) Correct :     We are given that            a + b + c + d =  0

                          So,                                  a + b = - (c + d)

                          Thus magnitude of a + c is equal to the c+d.

(c) Correct :-    We have              a + b + c + d =  0 

                                                          b + c + d = - a 

                            So clearly magnitude of a cannot be greater than the sum of the other three vectors.

(d) Correct:-   Sum of three vectors is zero if they are coplanar.

                         Thus,                        a + b + c + d =  0

                           or                             a + (b + c) + d =  0 

                          Hence (b+c) must be coplanar with a and d

Posted by

Devendra Khairwa

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