设D={(x,y)|x^2+y^2≤a^2},当a=?时,∫∫√a^2-x^2-y^2dxdy=π

问题描述:

设D={(x,y)|x^2+y^2≤a^2},当a=?时,∫∫√a^2-x^2-y^2dxdy=π