Mathematics IB Tutorial 2
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Mathematics IB Tutorial 2
1. Consider the differential equation
dy
dt
+ t y = t, y(0) = 0, using
(a) solve this using an integrating factor, and (b) approximate y(2) using 4 steps of Eulers method
and compare answers.
(In fact this is also separable so you can try solving that way as well).
2. (a) Write down a 3 x 3 matrix whose null space is
(i) a point; (ii) a line; (iii) a plane; (iv) R3 .
(b) Write down all 2 x 2 matrices whose null space is the line y = 2x.
Practice Questions
Algebra
1. Let V and W be subspaces with V S W . Show that dim V < dim W with equality if and only if V = W .
2. Let A = ← .
(a) Find a basis for Row A and Nul A.
(b) Is 2--1(4) in either of these?
(c) What are dim Row A and dim Nul A?
3. Find bases for the row space and null space for each of the following matrices.
┌ 1 1 1 -1 0┐
B = ┌ ┐
'-1 -2 3' .
5. Find bases for the row space and null space for each of the following matrices.
(a)
┌┐
A = 1 0 0 1
'5 6 8 5'
(b)
┌ ┐
B = -1 1 -1
( 0 -1 0 )
6. (a) If A is an m x n matrix, show that v e Row(A) if and only if v = xtA for some x e Rm .
(b) Using the above result, show that Row(A + B) S Row(A) + Row(B). (Recall that for two subspaces U , V of R| we define U + V := {u + v | u e U, v e V }, which is also a subspace of R| .)
7. Use an example to show that if B is not invertible then it may be that Row BA Row A.
8. [Non- examinable challenge] Show that Nul(AtA) = Nul(A) for every matrix A.
Calculus
9. Solve the initial value problem
t2y\ + 2ty = ln t , y(1) = 2 .
10. Solve each of the following differential equations.
(a) = t2 (1 - 2y) (b) + y cot t = 2 sin t, y(π/2) = 1
11. Solve ty + 2y\ = 4t2 subject to y(1) = 2.
12. Solve for u(x) the DE
du
dx
13. Find the general solution to
14. Find the general solution to
+ y = cos ^t .
(Note: you will not be able to perform the final integral, and so will need to express your solution in terms of an integral.)
15. A reservoir contains 10 x 106 litres of fresh water. A stream, with a flow rate of 5 x 106 litres per year, brings water into the reservoir, while well-mixed water is let out of the reservoir at exactly the same rate, thereby maintaining a constant volume of water in the reservoir. On April 1 (we will set this to be time 0), a factory’s output begins to contaminate the incoming stream, such that the incoming stream contains γ(t) = 2 + sin (2t) grams of mercury per litre (this varies periodically with time t).
(a) Construct a differential equation model for the amount of mer-
cury M (t) (in grams) in the reservoir at a general time.
(b) Solve your model, thereby determining the amount of mercury
in the reservoir at a general time t. What is your long-term prediction for what happens?
16. [Non- examinable challenge] If a e R is a constant, and g is an arbitrary continuous function, find the general solution to
a
t
17. Consider the DE
y\ = 0.5 - t + 2y , y(0) = 1 .
(a) Find the approximate numerical value for y(0.4) by using the
Euler method with ∆t = 0.1.
(b) Repeat with ∆t = 0.05, and compare your results with (a).
(c) Repeat with ∆t = 0.025, and compare with (a) and (b).
(d) Find the explicit solution y = φ(t) using a relevant exact tech- nique, and compare φ(0.4) with the answers you got from the previous parts. Check what is approximately happening to the error as ∆t is halved (for example, is it halving too?), and hence validate what the course notes say is the order of Euler’s method.
18. Using the Euler method, find approximate numerical solutions to the differential equations, and plot their solutions. Test your values of the step size until you find one which appears to give good solutions in that y at the final value no longer changes to two decimal places if the step size is reduced.
dy
dx
(b) y\ + t2y3 = t ; ; y(0) = 1 ; t e [0, 1]
19. Use Matlab’s built-in Runge-Kutta methods to find solutions to the equations in the previous problem.
20. Write down the appropriate Euler method for numerically solving the differential equation
dy
dt
Are there initial conditions y1 for which you will be unable to imple- ment your algorithm?
21. [Non- examinable challenge] Use the Euler and Runge-Kutta methods to find numerical approximations to
dy 1/3
(a) Do the numerical solutions that you get seem correct? In other
words, do you believe that they do indeed solve the differential equation?
(b) Use the method of separation of variables to solve the differential
equation. Did you find a solution which seems at odds with what you have got in previous parts? Can you guess why?
2022-09-02