已知向量\( \overset{→}{a}, \overset{→}{b} \)的夹角\(\left| \overset{→}{a}\right|=1 \)为\(120^{\circ} \)，且，\(\left| \overset{→}{b}\right|=2 \)，则向量\( \overset{→}{a}+ \overset{→}{b} \)在向量\( \overset{→}{a} \)方向上的投影是