在平行四边形ABCD中,\( \overrightarrow {AB}\)=\( \overrightarrow {a}\),\( \overrightarrow {AD}\)=\( \overrightarrow {b}\),\( \overrightarrow {AN}\)=3\( \overrightarrow {NC}\),M为BC的中点,则\( \overrightarrow {MN}\)=
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(\( \overrightarrow {a}\),\( \overrightarrow {b}\)表示)