M_y:=\int_{\cos\theta}^0\int_{x\newcommand{\tg}{\mathop{\mathrm{tg}}}\tg\theta}^{\sqrt{1-x^2}}y\,\hbox{d}y\,\hbox{d}x+\int_0^1\int_0^{\sqrt{1-x^2}}y\,\hbox{d}y\,\hbox{d}x={1-\cos\theta\over 3}