Question

A = 5, B = 4 & angle between Ā & B is 60° then A - B will be
(A) 5
(B) 4
(C) √21
(D) none of these


here A and B are magnitude of a vector.​

Answer 1

| A | = 5

| B | = 4

Here,

==> A - B

==> A + ( - B)

So angle between them would be:

==> θ = 180 - 60

==> θ = 120°

Also,

R = A + ( - B )

R² = | A |² + | B |² + 2ABcos θ

R² = 5² + 4² + 2×5×4×cos 120°

R² = 25 + 16 + 40×-1/2

R² = 25 + 16 - 20

R² = 25 - 4

R² = 21

Hence, | R | = √21

Option C is correct.