I'm not sure where you'd ask vector questions, but since this one involved $\displaystyle \cos$ I decided to ask it here:

Two vectors, a and b, have a common starting point with an angle of 120 degrees between them. The vectors are such that |a| = 3 and |b| = 4.

Calculate |a+b|

The way I thought of it was |a+b| = \sqrt{3^2+4^2-2(3)(4)(\cos{60})} = \sqrt {13}

However, the answer in the book is \sqrt {3}