二重积分积分区域为|cos(x+y)|其中d:0≤x,y≤二分之π？

原式=∫<0,π/2>dx∫<0,π/2-x>cos(x+y)dy
+∫<0,π/2>dx∫<π/2-x,π/2>-cos(x+y)dy
==∫<0,π/2>(1-sinx)dx-∫<0,π/2>(cosx-1)dx
=(2x+cosx-sinx)|<0,π/2>
=π-2