코사인함수 주기

함수 $f \left( x \right) = 6 \cos ax + 10$에서 주기가 $\displaystyle \frac{2 \pi}{a}$이므로 $\displaystyle \frac{2 \pi}{a} = 4 \pi$, $\displaystyle a = \frac{1}{2}$ $\displaystyle f \left( \frac{4}{3} \pi \right)$$\displaystyle = 6 \cos \left( \frac{1}{2} \times \frac{4}{3} \pi \right) + 10$ $\displaystyle = 6 \cos \frac{2}{3} \pi + 10$ $\displaystyle = 6 \cos \left( \pi - \frac{\pi}{3} \right) + 10$ $\displaystyle = - 6 \cos \frac{\pi}{3} + 10$ $\displaystyle = - 6 \times \frac{1}{2} + 10$ $= 7$