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1)

lf a and b are vectors such that |a+b|=29 and a×(2i+3j+4k)=(2i+3j+4k)×b the  a possible value of (a+b).(7i+2j+3k) is 


A) 0

B) 3

C) 4

D) 5

Answer:

Option C

Explanation:

 Concept involved  if a x b= a x c

   a x b-a  x c=0

a x (b-c)=0 ie, a || (b-c) 

 or b-c= λ a

 Sol. Here, a×(2i+3j+4k)=(2i+3j+4k)×b 

   a×(2i+3j+4k)(2i+3j+4k)×b

   (a +b) x (2i+3j+4k)=0

   a+b=λ(2i+3j+4k).....(i)

 since,  |a+b|=29

  ±λ4+9+16=29

     λ=±1

       a+b=±(2i+3j+4k)

 Now, (a+b)(-7i+2j+3k)

 =±(14+6+12)=±4