Cho Cos a = 1/5 ( 3π/2 < a < 2π ) Tính Sin2a, Cos2a, Tan2a

Ta có:

Sin²a = 1 - Cos²a = 1 - ($\frac{1}{5}$)² = $\frac{19}{25}$

Cos²a = Cos²a - Sin²a = ($\frac{1}{5}$)²  - ($\frac{19}{25}$ )² = $\frac{-82}{625}$

Tan²a = $\frac{Sin²a}{Cos²a}$= $\frac{\frac{19}{25} }{\frac{-82}{625} }$ = $\frac{-19}{82}$

Kết luận:

Sin²a = $\frac{19}{25}$, Cos²a = $\frac{-82}{625}$, Tan²a = $\frac{-19}{82}$

@DangThiBichNgan