原式=(2(1-cosa))^(1/2)-2(1+sina)^(1/2)=(2(1-(1-2sin^2(a/2))))^(1/2)-2(sin^2(a/2)+cos^2(a/2)+2sin(a/2)cos(a/2))^(1/2)=(4sin^2(a/2))^(1/2)-2((sin(a/2)+cos(a/2))^2)^(1/2)2.5pi