Question

If the projection of 5i - j - 3k on the vector i + 3j + λk is same as the projection of i + 3j + λk on 5i - j - 3k then λ is equal to …

(a) ± 4 

(b) ± 3 

(c) ± 5

(d) ± 1

Answer 1

Answer is (c) ± 5

Let vector a = 5i - j - 3k and vector b = i + 3j + λk projection of vector (a and b) = vector(a.b)/vector |b|  

Given (i) = (ii)