问题
下面【三角函数 $\cos$ 的特殊角数值】中，正确的是哪个？选项
[A]. $\cos 0$ $=$ $1$ $\color{White}{//}$ $\cos \frac{\pi}{6}$ $=$ $\frac{\sqrt{3}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{4}$ $=$ $\frac{\sqrt{2}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{3}$ $=$ $\frac{1}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{2}$ $=$ $1$[B]. $\cos 0$ $=$ $0$ $\color{White}{//}$ $\cos \frac{\pi}{6}$ $=$ $\frac{\sqrt{3}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{4}$ $=$ $\frac{\sqrt{2}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{3}$ $=$ $\frac{1}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{2}$ $=$ $1$
[C]. $\cos 0$ $=$ $1$ $\color{White}{//}$ $\cos \frac{\pi}{6}$ $=$ $\frac{\sqrt{3}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{4}$ $=$ $\frac{\sqrt{2}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{3}$ $=$ $\frac{1}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{2}$ $=$ $0$
[D]. $\cos 0$ $=$ $1$ $\color{White}{//}$ $\cos \frac{\pi}{6}$ $=$ $\frac{1}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{4}$ $=$ $\frac{\sqrt{2}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{3}$ $=$ $\frac{\sqrt{3}}{2}$ $\color{White}{//}$ $\cos \frac{\pi}{2}$ $=$ $0$
$\cos 0$ $=$ $1$ $\color{Red}{//}$ $\cos \frac{\pi}{6}$ $=$ $\frac{\sqrt{3}}{2}$ $\color{Red}{//}$ $\cos \frac{\pi}{4}$ $=$ $\frac{\sqrt{2}}{2}$ $\color{Red}{//}$ $\cos \frac{\pi}{3}$ $=$ $\frac{1}{2}$ $\color{Red}{//}$ $\cos \frac{\pi}{2}$ $=$ $0$