Question

Which of the following number is rational?

• A. $\sin {15}^o$
• B. $\cos {15}^o$
• C. $\sin {15}^o\cos {15}^o$
• D. $\sin {15}^o\cos {74}^o$

Answer 1

The correct option is C $\sin {15}^o\cos {15}^o$

$\sin {15}^o\cos {15}^o=\frac{1}{2}\times \left(2\sin {15}^o\cos {15}^o\right)=\frac{1}{2}.\sin {30}^o=\frac{1}{2}\times \frac{1}{2}=\frac{1}{4}$
Option C