# Chapter 5 - Section 5.4 - Product-to-Sum and Sum-to-Product Formulas - Exercise Set - Page 689: 20

The required solution is $2\sin \left( {{45}^{\circ }} \right)\sin \left( {{30}^{\circ }} \right)$, $-\frac{\sqrt{2}}{2}$.

#### Work Step by Step

One of the sum-to-product formula is $\cos \alpha -\cos \beta =-2\sin \frac{\alpha +\beta }{2}\sin \frac{\alpha -\beta }{2}$. So, in this question, according to the above-mentioned formula, the value of $\alpha$ is ${{75}^{\circ }}$ and the value of $\beta$ is ${{15}^{\circ }}$. Now, the expression can be evaluated as provided below: \begin{align} & \cos {{75}^{\circ }}-\cos {{15}^{\circ }}=-2\sin \left( \frac{{{75}^{\circ }}+{{15}^{\circ }}}{2} \right)\sin \left( \frac{{{75}^{\circ }}-{{15}^{\circ }}}{2} \right) \\ & =-2\sin \left( \frac{{{90}^{\circ }}}{2} \right)\sin \left( \frac{{{60}^{\circ }}}{2} \right) \\ & =-2\sin \left( {{45}^{\circ }} \right)\sin \left( {{30}^{\circ }} \right) \end{align} Thus, the values as per the trigonometry table are put to solve the expression further and to find the exact value: \begin{align} & -2\sin \left( {{45}^{\circ }} \right)\sin \left( {{30}^{\circ }} \right)=-2\left( \frac{\sqrt{2}}{2} \right)\left( \frac{1}{2} \right) \\ & =-\frac{\sqrt{2}}{2} \end{align} Hence, the provided expression can be written as $2\sin \left( {{45}^{\circ }} \right)\sin \left( {{30}^{\circ }} \right)$. And the product’s exact value is $-\frac{\sqrt{2}}{2}$.

After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.