# Homework Help: Finding the sum vector given only the moduli and angle

1. Jan 25, 2014

### K.QMUL

1. The problem statement, all variables and given/known data
Given two 2-dimensional vectors $\overline{a}$ and $\overline{b}$ of moduli l$\overline{a}$l = 3u and l$\overline{b}$l = 4u, and forming an
angle  of 120 degrees between them, determine the modulus of the sum vector $\overline{s}$ = $\overline{a}$ + $\overline{b}$
and the angle between $\overline{s}$ and $\overline{a}$.

2. Relevant equations

a.b = l$\overline{a}$ll$\overline{b}$lcosθ

3. The attempt at a solution

All if have calculated is the dot product of vectors a and b, coming to (6u2). I cannot seem to figure out what I can do to find the sum of these vectors only given the moduli and angle between them. Please Help :-)

2. Jan 25, 2014

### K.QMUL

I think I have solved my own question. I think I have to use the cosine rule and not the vector product rule as stated above

3. Jan 25, 2014

### SammyS

Staff Emeritus