## anonymous 3 years ago Write the expression as either the sine, cosine, or tangent of a single angle.

1. anonymous

2. sirm3d

the problem resembles that of a cosine of a difference of two angles

3. sirm3d

$\cos (A-B)=\cos A \cos B + \sin A \sin B$

4. anonymous

so is it cosin(pi/5 -pi/7)?

5. anonymous

@sirm3d?

6. sirm3d

that's right.

7. sirm3d

you should combine pi/5-pi/7 into one angle, as required in the problem.

8. anonymous

cos π/7?

9. sirm3d

$\frac{\pi}{5}-\frac{\pi}{7}=\frac{7\pi-5\pi}{35}=\frac{2\pi}{35}$