Chứng minh (1 + 2cosx + cos2x)/sinxcosx * (cosx + 1) = 1/2sinx; Rút gọn B = cos^2x + cos^2(x - pi/3) + cos^2(x - 2pi/3)

a) Chứng minh (1 + 2cosx + cos2x)/sinxcosx * (cosx + 1) = 1/2sinx
b) Rút gọn B = cos^2x + cos^2(x - pi/3) + cos^2(x - 2pi/3)