4. (5 分) 若 $\sin a=\frac{1}{3}$, 则 $\cos 2 a=(\quad)$
Gaokao · Math · previous-year question
- A.$\frac{8}{9}$
- B.$\frac{7}{9}$
- C.$-\frac{7}{9}$
- D.$-\frac{8}{9}$
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