If C is the plane triangle which is enclosed by the lines \(x = \frac{\pi }{2},\;y = \frac{2}{\pi }x\) and y = 0, then the value of following integral \(\mathop \smallint \nolimits_C \left[ {\cos x\;dy + \left( {y - \sin x} \right)dx} \right]\) comes out to be \(\frac{\pi }{a} + \frac{b}{\pi }\). Find |ab|