Take the derivative of cos( + ) with respect to , thinking of as a constant.

Take the derivative of cos(θ + ϕ) with respect to θ, thinking of ϕ as a constant. Simplify the answer in terms of sin(θ + ϕ).
Use the angle addition formulas (Equations 2.10.1 and 2.10.2) to expand the following and use the product and sum rules to compute the derivatives of the above functions. Compare the result with what you get with the chain rule.