Question

Rút gọn A=cos(pi/3+a)+cos(pi/3-a)

Answer 1

\(A=cos\left(\dfrac{\pi}{3}+\alpha\right)+cos\left(\dfrac{\pi}{3}-\alpha\right)\)

\(=cos\dfrac{\pi}{3}.cos\alpha-sin\dfrac{\pi}{3}.sin\alpha+cos\dfrac{\pi}{3}.cos\alpha+sin\dfrac{\pi}{3}.sin\alpha\)

\(=2.cos\dfrac{\pi}{3}.cos\alpha=cos\alpha\)

Answer 2

`A=cos(π/3 +α) + cos(π/3-α)`

`=cos π/3 . cos α - sin  π/3 . sinα + cos π/3 . cosα + sin π/3 . sinα`

`=2 . 1/2 . cos α`

`= cosα`