Cho biết $latex \displaystyle \text{cos}\alpha \,\,\text{=}\,\,\frac{1}{3}$. Tính giá trị $latex \displaystyle \text{cos}2\alpha $
A. $latex \displaystyle \text{cos}2\alpha =\frac{7}{9}$
B. $latex \displaystyle \text{cos}2\alpha =\frac{1}{3}$
C. $latex \displaystyle \text{cos}2\alpha =-\frac{7}{9}$
D. $latex \displaystyle \text{cos}2\alpha =\frac{2}{3}$
Đáp án
Câu C