MATH0046 examination summer 2021
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MATH0046 examination summer 2021
There are 5 questions and you should attempt them all. Each question carries equal credit.
1. Let C1 denote the circle x2 + y2 = 1 and consider the function
F (x, y) = x3 (y - 1)2 .
(a) Suppose points are constrained to lie on C1 . Find the coordinates of all the constrained stationary points of F and identify which, if any, are constrained maxima or minima.
(b) Consider the region D that lies within C1 and also satisfies y > 0. Find the maximum and minimum of F over the region D (including its boundaries), and give the coordinates at which these values occur.
(c) Now let Cγ denote the circle x2 +y2 = γ 2 for some γ e R, and suppose points are constrained to lie on Cγ . Let (x* , y* ) denote the coordinates of the maximum value of F subject to this constraint. By calculating these coordinates when γ is large, show that the ratio of x* and y* approaches a constant value α (independent of γ) as γ - o,
*
*
and determine α .
2. (a) Find the solution of each of the following differential equations:
(i)
dy
dx
(ii)
dy y
dx x
(b) Consider the following non-linear system of differential equations:
dx
= -(2x2 + 6y + 5)x.
dt
Write down an expression for dy/dx. Use the substitution u = x2 + 3y to deter- mine an implicit relationship between x(t) and y(t). Sketch some trajectories on an x - u plane. Hence show that, if y = 1 when x = 0, x(t) must remain bounded for all time by -a ≤ x(t) ≤ a for some a that you should determine.
3. Consider the quadratic form
Q(α) = ╱ -2x1(2) + 2x2(2) - 8x1 x3 - 8x2 x3、,
where α = (x1 , x2 , x3 ).
(a) Write down a symmetric matrix A such that Q = α尸 Aα where α = (x1 , x2 , x3 ), and determine its eigenvalues. Hence, or otherwise, re-write Q in the form Q = ay 1(2) + by2(2) + cy3(2) for some coefficients a, b and c and new coordinates 』(α) = (y1 , y2 , y3 ).
(b) Show that Q = 0 whenever α points in the direction of 0 , for some unit vector 0 that you should determine.
(c) If llαll = γ > 0, determine the maximum value Qmax of Q(α) as a function of γ, and give any vector α for which this maximum is attained (i.e. for which Q(α) = Qmax ).
(d) Let S denote the set of vectors α that satisfy both llαll = γ and Q(α) = Qmax . You have already found one element of S in part (c). Show that S contains more than one vector, and give the most general parameterisation for an element of S .
4. Consider the following linear system of differential equations,
= 8x + 5y
dt
dy
for some function f (t).
(1)
(2)
(a) In the case f = 0, find the general solution and sketch some trajectories on the x - y plane, showing how solutions evolve over time.
(b) Suppose now f (t) = -2 + 15t. Determine the solution in this case if x(0) = 0 and y(0) = 1.
(c) Consider the related inhomogeneous 2nd-order ODE
d2 z dz
dt2 dt
where z˙ = dz/dt. By relating the original system of equations to this new 2nd-order equation, use your answer to part (b) to determine the solution z(t).
5. (a) Consider a unit half-sphere occupying the region (x2 +y2 +z2 )1/2 ≤ 1 and z > 0. A ‘dog-bowl’ shape is created by removing the conical region z > λ(x2 + y2 )1/2 , for some λ > 0.
(i) Calculate the volume of this dog-bowl shape as a function of λ, leaving your answer in a form that does not involve any trigonometric functions.
(ii) For what value of λ is the volume of the dog-bowl shape the same as the volume of the conical section that was removed?
(b) Let A denote the semi-infinite strip described by x - 1 ≤ y ≤ x + 1 and x > 0. Consider the integral
xα (y - x)n
A (1 + x)2α+2
where α > -1 is a real number and n is a positive integer.
(i) Use the transformation u = y - x to rewrite I as an integral over a semi- infinite rectangular domain. Hence show that I = 0 if n is an odd integer.
(ii) Assuming n is an even integer, use another substitution to determine I(α, n) in terms of the beta function.
(iii) Relate your answer to the gamma function, and thus evaluate I(3/2, n) for even integers n, simplifying your answer as much as possible.
2022-05-09