This English Calculus I lecture develops the six trigonometric derivative formulas from the sine and cosine rules, then works a product-rule derivative, a tangent line, and a particle-at-rest problem. It also explains why the formulas assume radian measure and highlights the sign, domain, and rule-selection errors that commonly lose exam credit. The standard formulas and worked-problem patterns follow OpenStax Calculus Volume 1, Section 3.5. 1