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EART40005 – MATHS METHODS 1 This examination paper comprises 6 questions. Answer ALL questions. The total marks for this exam are 50. This time-limited remote assessment is being run as an open-book examination. We have worked hard to create exams that assesses synthesis of knowledge rather than factual recall. Thus, access to the internet, notes or other sources of factual information in the time provided will not be helpful and may well limit your time to successfully synthesise the answers required. Where individual questions rely more on factual recall and may therefore be less discriminatory in an open book context, we may compare the performance on these questions to similar style questions in previous years and we may scale or ignore the marks associated with such questions or parts of the questions. The use of the work of another student, past or present, constitutes plagiarism. Giving your work to another student to use may also constitute an offence. Collusion is a form of plagiarism and will be treated in a similar manner. This is an individual assessment and thus should be completed solely by you. The College will investigate all instances where an examination or assessment offence is reported or suspected, using plagiarism software, vivas and other tools, and apply appropriate penalties to students. In all examinations we will analyse exam performance against previous performance and against data from previous years and use an evidence-based approach to maintain a fair and robust examination. As with all exams, the best strategy is to read the question carefully and answer as fully as possible, taking account of the time and number of marks available.
– 2 – Candidates should answer ALL questions. 1. Consider the first-order, ordinary differential equation: dh dt = −h τ . Where τ is a constant. (a) Using the method of separation of variables, or otherwise, find the function h(t) that satisfies this equation, given the initial condition that h = −25 when t = 0. (4 marks) (b) This differential equation describes, approximately, the rebound of the crust after removal of an ice sheet; h represents the surface elevation in metres and t represents the time in years. In this case, describe the meaning of the constant τ . (2 marks) 2. Consider the matrix equation Ax = b, where A, x and b are given by: A = 0 2 41 1 2 2 6 3 , x = xy z , b = 4022 52 . (a) The matrix equation Ax = b represents three linear equations. Write out these equations explicitly, without using matrix notation. If x, y and z are Cartesian coordinates, what do these three equations represent? (2 marks) (b) Show that the determinant of A, detA = 18. What does this imply about the system of equations and its geometric interpretation? (3 marks) (c) Using Cramer’s rule to find at least one variable, solve the system of equations. Show and explain each step of your calculation. (5 marks) 3. Consider the infinite power series: 1 1− x = 1 + x+ x 2 + x3 + . . . (a) By differentiating the power series, or otherwise, find the power series for the function: 1 (1− x)2 . (2 marks) (b) Write down an expression for the kth term in the power series you found in Part 3(a). (2 marks) (c) For what range of x does the power series you found in Part 3(a) converge? Justify your answer. (4 marks) PAPER CONTINUED ON NEXT PAGE – 3 – (0,5) (3,4)(-4,3) x y (0,0) A B C Figure 1: 4. Consider the triangle ABC in Figure 1 above. (a) Convert the Cartesian co-ordinates of A, B and C to Polar co-ordinates. What is signifi- cant about the position of these points relative to the origin? (4 marks). (b) By considering the position vectors of points A, B and C, or otherwise, find the vectors a = −→ CA and b = −−→ CB. (2 marks). (c) By calculating a× b or otherwise, calculate the area of triangle ABC. (2 marks). 5. Evaluate the double integral: ∫ 1 0 ∫ 1 0 2xy2exy dx dy. Clearly show all steps in your solution. (8 marks) 6. Consider the scalar field φ(x, y, z) = C(x2 + y2 + z2) in a 3D Cartesian coordinate system. (a) Calculate the gradient vector field ∇φ. (3 marks) (b) If C = 23piGρ, where pi, G and ρ are constants, show by finding the divergence of ∇φ, or otherwise, that φ satisfies the Poisson equation ∇2φ = 4piGρ. (4 marks) (c) If φ is the gravitational potential within a uniform spherical body, centred at the origin (0, 0, 0), and g = −∇φ is the gravitational acceleration vector field, use your answer to Part 6(a), with constant C = 23piGρ, and your knowledge of spherical coordinates to derive an equation for how gravitational acceleration varies as a function of radius r inside the body. Use a sketch to illustrate your answer.