div class=trans-pagebuttonPage 1button div class=trans-image amp-img class=trans-thumb alt=Page 1: localtmpimage-00013 Either integrate by parts or use Fubinis theorem: r—l x PX x dx r f y dy clx r—l rx dx dy — yr f y dy An alternative proof is as follows src=https:reader034fdocumentsusreader034viewer20220424135f2de16244c71828ce0f44a5html5thumbnails1jpg width=142 height=106 layout=responsive amp-img divdivdiv class=trans-pagebuttonPage 2button div class=trans-image amp-img class=trans-thumb alt=Page 2: localtmpimage-00013 Either integrate by parts or use Fubinis theorem: r—l x PX x dx r f y dy clx r—l rx dx dy — yr f y dy An alternative proof is as follows src=https:reader034fdocumentsusreader034viewer20220424135f2de16244c71828ce0f44a5html5thumbnails2jpg width=142 height=106 layout=responsive amp-img divdiv