# Trigonometry! 40

Geometry Level 4

In $$\Delta ABC$$, find $$c$$, given $$a=5$$, $$b=4$$ and $$\cos (A-B) = \frac {31}{32}$$.

$$a$$, $$b$$ and $$c$$ are the sides of the triangle opposite to angles $$A$$, $$B$$ and $$C$$.

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