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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