Please reason step by step, and put your final answer within \boxed{}.
Given three position vectors $\overrightarrow {a}, \overrightarrow {b}, \overrightarrow {c}$ such that $\overrightarrow {a}= \overrightarrow {b}+ \overrightarrow {c}$, find the angle between vectors $\overrightarrow {a}$ and $\overrightarrow {b}$.
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