The diagram shows a triangle $$\(O A B\)$$ and a straight line $$\(O A C\)$$. $$\( OA: O C=2: 5 \)$$ and $$\(M\)$$ is the midpoint of $$\(A B\)$$. $$\(\overrightarrow{O A}=\mathbf{a}\)$$ and $$\(\overrightarrow{O B}=\mathbf{b}\)$$. Find, in terms of $$\(\mathbf{a}\)$$ and $$\(\mathbf{b}\)$$, in its simplest form $$\(\overrightarrow{A B}\)$$, $$\[ \overrightarrow{A B}= ............................................. \]$$

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