问答题 设F(x,y)在圆域x2+y2≤R2有连续偏导数,又f(x,y)|x2+y2=R2=0,
(Ⅰ) 设a>0,记Da=(x,y)|a2≤x2+y2≤R2,将化为定积分;
(Ⅱ) 求