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MAST20009 Vector Calculus
Writing time: 3 hours
Reading time: 15 minutes
This is NOT an open book exam
This paper consists of 5 pages (including this page)
Authorised Materials
• Mobile phones, smart watches and internet or communication devices are forbidden.
• No written or printed materials may be brought into the examination.
• No calculators of any kind may be brought into the examination.
Instructions to Students
• You must NOT remove this question paper at the conclusion of the examination.
• There are 12 questions on this exam paper.
• All questions may be attempted.
• Marks for each question are indicated on the exam paper.
• Start each question on a new page.
• Clearly label each page with the number of the question that you are attempting.
• There is a separate 3 page formula sheet accompanying the examination paper, which you
may use in this examination.
• The total number of marks available is 151.
Instructions to Invigilators
• Students must NOT remove this question paper at the conclusion of the examination.
• Please supply graph paper.
• Initially students are to receive the exam paper, the 3 page formula sheet, two 11 page
script books and two sheets of graph paper.
This paper may be held in the Baillieu Library
Blank page (ignored in page numbering)
MAST20009 Semester 2, 2019
Question 1 (12 marks)
Prove that the function
f(x, y) =
x
4 sin
(
1
x2 + |y|
)
, (x, y) 6= (0, 0)
0, (x, y) = (0, 0)
is continuous on all of R2.
Question 2 (9 marks)
For each of the following statements about a given scalar valued function f in two variables x
and y, decide whether it is true or false. Give brief justifications for your answers.
(a) If the partial derivatives of f both exist and are continuous, then f is continuous.
(b) Wherever the second derivatives of f are defined, we have
∂2f
∂x∂y
=
∂2f
∂y∂x
.
(c) If f is C1 then f is also C2.
Question 3 (12 marks)
(a) State the matrix chain rule. Be careful to include the conditions under which it holds and
to explain each symbol you use.
(b) Let
f(u, v, w) = (eu−w, sin(u+ v + w))
and let
g(x, y) = (ex, cos(y − x)).
Calculate g ◦ f and, using the matrix chain rule, calculate D(g ◦ f)|(0,0,0).
Question 4 (20 marks)
(a) Use Lagrange multipliers to determine the maximum of the function f(x, y) = 4xy + y
restricted to the line segment given by x+ y = 1 and x ≥ 0 and y ≥ 0.
(b) Prove that the answer you found in part (a) is indeed a maximum.
(c) On the graph paper provided, draw a high quality picture of this situation: the constraint,
at least three level curves of f , the gradient of f at three points and the gradient of the
constraint function g at three points. Pay attention to labels and scale.
Page 2 of 5 pages
MAST20009 Semester 2, 2019
Question 5 (16 marks)
(a) Sketch the path
γ(t) =
t cos(tpi)t sin(tpi)
t
t ∈ [0, 6]
in R3 and describe the journey of a particle along this path (interpreting t as time variable)
in words.
(b) Compute the tangent, normal and binormal vectors of the path
c(t) =
2 sin(t)2−√2 cos(t)√
2 cos(t)
t ∈ R.
(c) Compute the curvature and the torsion of c.
(d) Interpret your answers to (b) and (c) geometrically.
Question 6 (12 marks)
Consider the vector field
~F (x, y) =
[
2y
−2x
]
.
(a) Use the graph paper provided to draw the vector field at the points (1, 1), (−1, 2), (2,−1),
(−3,−1) and (0,−1).
(b) Make an educated guess about the flow lines of ~F .
(c) Determine the equation for the flow line of ~F passing through the point (1, 1) in terms of
x and y. Show your working.
Question 7 (6 marks)
(a) Let f be a scalar valued function in three variables x, y and z. Under which condition on
f does the gradient of f satisfy the vector identity
∇× (∇f) = ~0?
[Hint: It is possible to figure this out.]
(b) Prove the identity in part (a), assuming the condition you found for f .
Page 3 of 5 pages
MAST20009 Semester 2, 2019
Question 8 (8 marks)
Consider the double integral ∫ 2
0
∫ x
0
x2y dy dx.
(a) Sketch the region of integration.
(b) Evaluate the integral in the form given.
(c) Change the order of integration and evaluate the integral again.
Question 9 (6 marks)
(a) Use polar coordinates to parametrize the interior of the ellipse with major axis of radius
a along the x-axis and minor axis of radius b along the y-axis.
a
b
y
x
(b) Using double integrals and your parametrization from part (a), prove that the volume
of the elliptical cylinder with semi-major axis a, semi-minor axis b, and height h equals
piabh.
Question 10 (10 marks)
Let D be the region bounded by the paraboloids z = x2 + y2 − 4 and z = 8− 2x2 − 2y2.
(a) Determine where the paraboloids intersect.
(b) Sketch the region D.
(c) Using cylindrical coordinates, evaluate the triple integral∫∫∫
D
x2 dV.
Page 4 of 5 pages
MAST20009 Semester 2, 2019
Question 11 (20 marks)
Let E ⊂ R2 be the ellipse with main radius equal to 5 and minor radius equal to 3, centred at
the origin with the main axis aligned with the x axis.
(a) Parametrize E using the counter-clockwise orientation and starting at the point (5, 0).
(b) Parametrize the rays from (−4, 0) to (0, 3) and from (−4, 0) to
(
5√
2
, 3√
2
)
.
(c) Using Green’s theorem in the plane, prove that the area of a closed bounded domain D
with boundary ∂D is given by
A(D) =
∫
∂D
x dy.
(d) Use your result from part (c) to calculate the area of the domain bounded by E and the
two rays in part (b).
(e) Using the graph paper provided, draw a high quality picture of the domain D and its
boundary, indicating orientations and paying attention to scale and labels.
Question 12 (20 marks)
Let S be the sphere of radius 6, centred at the origin.
(a) Give a parametrization
Ψ : [0, pi]× [0, 2pi] −→ S.
(b) Calculate the tangent vectors and the outward normal vector of S.
(c) Let S+ be the upper hemisphere, i.e., the part of S with z ≥ 0, oriented by the outward
normal vector. Let ~F be the vector field
~F (x, y, z) =
yz
1
on S+. Verify Stokes’ theorem for ~F and S+.
End of Exam—Total Available Marks = 151