|a+b|=根号(a+b)^2 = 根号(a^2+2ab+b^2) = 根号(3+ 2*2*根号3* cos30° +4) =根号(17/2)|a-b|=根号(a-b)^2 =根号(a^2-2ab+b^2) =根号(3-2*2*根号3* cos30° +4) =根号(11/2)
