Question 5

 

Hello Manjiri, given a, b and c are unit vectors. So |a| = |b| = |c| = 1
Now |a - b|^2 = 1 + 1 - 2 cos (180 - C)
And |b - c|^2 = 1 + 1 - 2 cos (180 - A)
Also |c - a|^2 = 1 + 1 - 2 cos (180 - B)
Now adding we have 6 + 2( cos A + cos B + cos C)
As A, B and C are to be 60 degree, we have 6 + 2 (3/2) = 9 (not less than 9)
Diagram would help you a lot
All the best