Đáp án đúng: A
Giải chi tiết:\(\eqalign{ & A = {(\cos a + \cos b)^2} + {(\sin a + \sin b)^2} = {\cos ^2}a + 2\cos a\cos b + {\cos ^2}b + {\sin ^2}a + 2\sin a\sin b + {\sin ^2}b \cr & = \left( {{{\cos }^2}a + {{\sin }^2}a} \right) + \left( {{{\cos }^2}b + {{\sin }^2}b} \right) + 2(\cos a\cos b + \sin a\sin b) \cr & = \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, + \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, + \,2\cos (a - b) \cr & = 2 + 2\cos {\pi \over 3} = 2 + 2.{1 \over 2} = 3 \cr} \)
Chọn: A