A simple model for the cost of a car journey £ C C\,C when a car is driven at a steady speed of v v\,v mph is
C=5000v+v+10 C = \frac{5000}{v} + v + 10 C=v5000+v+10For this model, v=5000v = \sqrt{5000}v=5000 mph (70.7 mph to 3 significant figures) is a stationary value.
Use d2Cdv2\displaystyle \frac{d^2C}{dv^2}dv2d2C to verify that C C\,C is a minimum for this value of vvv
264 exam-style questions on Edexcel A Level Maths Differentiation, covering 12.1 Gradients of Curves, 12.2 Differentiation from first principles, 12.3 Differentiating x^n, 12.4 Differentiating Quadratics, 12.5 Differentiating functions with two or more terms, 12.6 Gradients, Tangents and Normals, 12.7 Increasing and Decreasing Functions, 12.8 Second Order Derivatives, 12.9 Stationary Points, 12.10 Sketching Gradient Functions, and 12.11 Modelling with Differentiation. Each one has a worked solution and a mark scheme showing where the marks go.