Question

Let a = i + 2j + k, b = i – j + k and c = i + j – k. A vector in the plane of a and b, whose projection on c is 1/√3, is 

(a) 4i – j + k 

(b) 3i + j + k 

(c) 2i + j + 2k 

(d) 4i + j – 4k

Answer 1

Answer is (c) 2i + j + 2k

Explanation:

Let v be a vector.

A vector v in the plane of a and b

So, v, a, b are coplaner vectors. 

Thus,

Therefore, v = 2i + j + 2k