MATH2021: Vector Calculus and Differential Equations
Vector Calculus and Differential Equations
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MATH2021: Vector Calculus and Differential Equations
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0 Z No credit awarded.
1. Let C be the part of the graph of y =
√
1 + x2 between x = 0 and x = 1.
(a) Write down a parametrisation of C, and find the velocity vector of your parametrisation.
(b) Find the line integral ∫
C
xy ds.
2. Let F (x, y, z) = (1 + 2xyz3, z + x2z3, y + 3x2yz2).
(a) Show that curl(F) = 0.
(b) Find a function f : R3 → R such that F = ∇ f .
(c) Hence, or otherwise, find the value of the integral∫
C
F · d s,
whereC is the part of the helix with parametrisation γ(t) = (cos t,sin t, t) with t ∈ [0, pi].
3. (a) Write the iterated integral ∫ 1
0
∫ x
x2
f (x, y) dydx
as an iterated integral in the reverse order (that is, with inner integral with respect to x
and outer integral with respect to y). Include in your solution a sketch of the domain of
integration of the iterated integral.
(b) Let P be the parallelogram in R2 with vertices (0,0), (1,1), (4,−2), and (5,−1). Calcu-
late the double integral "
P
(x − y) dA.
4. Calculate the area enclosed by the curve
γ(t) = (4 cos t − cos(4t),4 sin t − sin(4t)), t ∈ [0,2pi].
A sketch of the curve is shown below. Hint: Green’s Theorem may help.
MATH1903 Integral Calculus 33
Example 3.19. An epicycloid is a curve traced out by a fixed point on the rim of a circle as
it rolls around on the outside of a larger circle. There is a good ill stration on the webpage
http://en.wikipedia.org/wiki/Epicycloid. If the radius of the rolling circle is 1 and the
radius of the larger circle is k then th corresponding epicycloid has parametric equation
x(t) = (k + 1) cos t− cos((k + 1)t)
y(t) = (k + 1) sin t− sin((k + 1)t) t ∈ [0, 2pi]. (3.1)
Some examples are shown in Figure 3.9.
−3 −2 −1 0 1 2
−3
−2
−1
0
1
2
3
−3 −2 −1 0 1 2 3
−4
−3
−2
−1
0
1
2
3
4
−5 0 5
−5
−4
−3
−2
−1
0
1
2
3
4
5
−6 −4 −2 0 2 4 6
−6
−4
−2
0
2
4
6
Figure 3.9: Epicycloids for k = 1 (cardioid), k = 2, 3 and k = 4
Compute the length of the epicycloid given by (3.1).
Solution. A similar calculation to the hypocycloid computation gives
x′(t)2 + y′(t)2 = 4(k + 1)2 sin2(kt/2).
Therefore
L = 2(k + 1)
∫ 2pi
0
| sin(kt/2)| dt = 2k(k + 1)
∫ 2pi/k
0
sin(kt/2) dt = 8(k + 1).
Remark 3.20. The epicycloid with one cusp (corresponding to k = 1) is called the cardioid .
Its parametric equation can also be written as
(x(t), y(t)) = (2(1 − cos t) cos t, 2(1 − cos t) sin t), t ∈ [0, 2pi].
Remark 3.21. In the Ptolemaic (Earth-centred) system of the solar-system the planets orbits
are epitrochoids. These are generalised versions of epicycloids, where the point is not necessarily
on the rim of the rolling circle.
Example 3.22. The Euler spiral has parametric equation in terms of the Fresnel integrals:
x(t) =
∫ t
0
cos(s2) ds, y(t) =
∫ t
0
sin(s2) ds, t ∈ R.
Write r(t) = (x(t), y(t)), and let τ (t) = r′(t)/‖r′(t)‖ be the unit tangent vector. The curvature
of C is, by definition, ‖τ ′(t)‖. Calculate the length of the Euler spiral between t = 0 and t = T .
Calculate the curvature of the Euler spiral at t = T . Hence show that the curvature of the Euler
spiral varies linearly with arc length.