If \( y=y(x),\ y\in\left[0,\dfrac{\pi}{2}\right] \) is the solution of the differential equation \( \sec y\,\dfrac{dy}{dx}-\sin(x+y)-\sin(x-y)=0 \), with \( y(0)=0 \), then \( 5y'\!\left(\dfrac{\pi}{2}\right) \) is equal to ________.