Let $P Q R$ be a triangle of area $\triangle$ with $a=2, b=\frac{7}{2}$ and $c=\frac{5}{2}$, where $a, b$ and $c$ are the lengths of the sides of the triangle opposite to the angles at $P, Q$ and $R$ respectively. Then $\frac{2 \sin P-\sin 2 P}{2 \sin P+\sin 2 P}$ equals
