Can someone please help explain this?
When designing a solar-energy collector, it is necessary to account for the latitude and longitude of the location, the angle of the sun, and the angle of the collector. In doing this, this equation is used: cos θ = cos A cos B cos C + sin A sin B. If θ = 90 degrees, show that cos C = - tan A tan B.
Hint: First evaluate the cosine of 90 degrees in the formula given. Then solve the equation for cos C. You should see something familiar appear. Use the identity for Tangent as sin/cos to finish it up.