Evaluate the double angle for cosine using a triangle

The video tutorial explains how to find the cosine of a double angle using different representations. The instructor recommends using the formula cosine squared minus sine squared for evaluation unless other options are more suitable for solving or verifying identities. An example calculation is provided, demonstrating the step-by-step process of using the preferred formula. The tutorial concludes with tips on handling these types of problems effectively.

How do you simplify the expression 16/25 - 9/25?

What is the most difficult aspect of the problems discussed?