Don't try to multiply!

Geometry Level 4

$\left( 1+\cos { \frac { \pi }{ 9 } } \right) \left( 1+\cos { \frac { 3\pi }{ 9 } } \right) \left( 1+\cos { \frac { 5\pi }{ 9 } } \right) \left( 1+\cos { \frac { 7\pi }{ 9 } } \right)$

The value of the expression above can be expressed as $\dfrac PQ$, where $P$ and $Q$ are coprime positive integers, find $Q-P$.

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