## mitchelsewbaran Write the expression as either the sine, cosine, or tangent of a single angle. one year ago one year ago

1. mitchelsewbaran

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. mitchelsewbaran

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

5. mitchelsewbaran

@sirm3d?

6. sirm3d

that's right.

7. sirm3d

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

8. mitchelsewbaran

cos π/7?

9. sirm3d

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