Differential Equations Homework Help

Differential Equations Homework Help
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Problem 1A function f (x) is deﬁned to be,
Problems
For −2π ≤ x ≤ 2π, sketch the graph of y = f (x). Do each of the following.
(i)Label all vertical asymptotes. Use the form “y = number”.
(ii)Label all local maxima and local minima (if any). Give the coordinates for each labelled
point.
(iii)Label all inﬂection points (if any). Give the coordinates and the derivative of each labelled
point.
(iv)Label each region where the graph is concave up. Label each region where the graph is concave
down.
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Problem 2 Figure 1 depicts two fixed rays L and M meeting at a fixed acute angle φ.
A line segment of fixed length c is allowed to slide with one endpoint on line L and one endpoint
on line M . Denote by a the distance from the origin of ray L to the endpoint of the segment on
line L. Denote by b the distance from the origin of ray M to the endpoint of the segment on line
M . Denote by θthe angle made by the line segment and the ray L at the point where they meet.
For the position of the line segment making b maximal, express a, b and θ in terms of the
constants: c and φ. The expression for the one remaining angle of the triangle is easier than the
expression for θ. You may want to start by computing that angle, and then solve for θ using that
the sum of the interior angles of a triangle equals π.
Show your work. You may find the law of cosines useful,
c2
= a2
+ b2
− 2ab cos(φ).
Problem 3 Solve Problem 17 from §4.4, p. 137 of the textbook. You are free to use any (valid)
method you like. You may find the following remarks useful.
Warning: This graph is trickier than it seems! Before attempting the problem, it may be helpful
to use a computer or graphing calculator to get an idea what the graph looks like.
Figure 2 depicts an isosceles triangle circumscribed about a circle of radius R. The two similar sides
each have length A + B, and the third side has length 2A. Express the area of each right

A A
B B
R R
 
 
 
R
A A
Figure 2: An isosceles triangle circumscribing a circle of
radius R
triangle in terms of either tan(α) or tan(β). Because the sum of the angles of a
equals π − 2α. Recall the doubleangle formula for tangents,
and the complementary angle formula for tangents,
tan(π − θ) = − tan(θ).
Using these, express tan(β), and thus the total area of the triangle, in terms of T = tan(α). Now
minimize this expression with respect to T , ﬁnd the corresponding angle α, and the height of the
triangle.

Solution (a) Since y = x + x−1, then y� = 1 − x−2. Therefore
We infer that there is a local maximum at x = −1, where y = −2, and a local minimum at x = 1 where y
= 2. Considering that y = f(x) almost equals x for large values of x, we are now ready to sketch its
graph, which is shown in Figure 1.
Solution (b) Let them intersect at the point (x0, y0). Because they intersect at (x0, y0)
This implies that tan ax0 = 1 which in turn implies ax0 = π/4 + nπ for some integer n. Therefore sin
ax0 = cos ax0 = ±1/ √2. Moreover, because the curves intersect at right angles
Solutions

in other words
a cos ax0.(−a sin ax0) = −1
or cos ax0 sin ax0 = ±1/ √2 = 1/a2, hence a = √2 since a > 0.
Solution (c)
which, when combined with the quotient rule, gives
We see that y= 0 for −3x2 +3 = 0, i.e x = ±1.Noting that y = 4 is a horizontal asymptote which y gets
arbitrarily close to as x → ±∞, we form the table below

We’re now ready to sketch the graph as in Figure
Figure 2: The description of the wall and ladder as in problem (d).

Solution (d)
The length of the ladder is given by
Then, we let
equal to zero, which gives the equation for the critical angle θ :
i.e.

Putting back into (1), we obtain
Solution (e) The weight of the sheet of the metal is proportional to its area, which is
A = πr2 + 2πrh
where r is the radius of the base and h the height. We want to maximize the volume of the can:
We express h from the (2)
therefore
By differentiating

Figure 3: A sketch of the circular pool in problem (d).
from which we obtain
Solution (f )
The time spent is
Differentiating and equating it to zero

hence θ = π/6 = 300.
Solution (g)
The length of the string is given by
where x is the horizontal distance between the boy and the kite.
Differentiating with respect to time, we obtain
We observe that x = 60 when L = 100. Putting this and the given dx = 20 ft/s in the last equation
yields
which is the speed of the string let out by the boy.
Solution (h)
We are given that the ice ball melts proportional to its area, in symbols

where V = 4πr3 is the volume and A = 4πr2 is the area of the ice ball with radius r. Rewriting 3 the
above equation and using the chain rule
we obtain
therefore
Now, at half the volume the radius is given by
The half volume is reached at t = 2, therefore using (3)
The radius r hits zero at time

Therefore, the additional time necessary to melt is,
Therefore, the additional time necessary to melt is,
2/(√
3
2 −1) hours.
Solution (i) We are given y = f (x) = x3 + 3x2 −6. Then
yJ
= f J
(x) = 3x2
+ 6x = 3x(x + 2)
We observe that f J(x) < 0 only for −2 < x < 0, otherwise f J(x) ≥ 0.
Some of its values are: f (−2) = −2, f (−1) = −4. f (0) = −6, f (1) = −2, f (2) = 14.
Now, since f (−2) < 0, and f J(x) > 0 for x < −2, we can say that f has no roots for x < −2.Also,
since f (−2) < 0, and f J(x) < 0 for −2 < x < 0, we can say that f has no roots for −2 < x
<
0.Moreover, since f (0) = −6, and f J(x) > 0 for x > 0, we can say that f has at most one root for
x > 0.
Furthermore, since f (1) = −2 < 0 and f (2) = 14 > 0, we conclude that f has exactly one
root which is between 1 and 2 by the intermediate value theorem.
Now, we are ready to calculate the Newton’s iterates

with the initial guess x1 = 1. Then
Repeating yields
Since x4 and x5 have the first the six digits the same, we conclude that x5 is at least accurate to six
digits.
Solution (j)
Therefore y = f(x) is always increasing, which implies it cannot have more than one root.
Since f(2) = 6 > 0, and f(1) = 4 < 0, by the intermediate value theorem, f has a root between 2 and 1.
Now, we are ready to calculate the Newton’s iterates mathhomeworksolver.com

with the initial guess x1 = 1. Then
Repeating yields
Since x5 and x6 have the ﬁrst the six digits the same, we conclude that x6 is at least
accurate to six digits.
Part II
Problem 1A function f (x) is deﬁned to be,

For −2π ≤ x ≤ 2π, sketch the graph of y = f (x). Do each of the following.
(i)Label all vertical asymptotes. Use the form “y = number”.
(ii)Label all local maxima and local minima (if any). Give the coordinates for each
labelled point.
(iii)Label all inﬂection points (if any). Give the coordinates and the derivative of each
labelled point.
(iv)Label each region where the graph is concave up. Label each region where the graph is
concave down.
Warning: This graph is trickier than it seems! Before attempting the problem, it may be
helpful to use a computer or graphing calculator to get an idea what the graph looks like.
Solution to Problem 1
Since the function y = f (x) is periodic with period 2π, it suﬃces to do the analysis for only
[0, 2π].
Solution to (i) The vertical asymptotes MAY be located at places where cos x = 0, which
happens

therefore there is no asymptote at x = π where the function is continuous.
Solution to (ii) By the quotient rule,
which is always nonnegative. Therefore f is nondecreasing and there are no local extrema.
Solution to (iii) By the chain rule
which changes sign whenever cos x = 0 which happens at x = π and 3π . However, there is a vertical
asymptote at x = π , therefore the only inﬂection point occurs when x = 3π . The coordinates of
this point is given by ( 3π , 0). The slope at the inﬂection point is f J( 3π ) = 1/2.
Solution to (iv) The function is concave up if cos x > 0 and concave down if cos x < 0. The
table below summarizes all we have mentioned:

A line segment of ﬁxed length c is allowed to slide with one endpoint on line L and one endpoint
on line M . Denote by a the distance from the origin of ray L to the endpoint of the segment on
line L. Denote by b the distance from the origin of ray M to the endpoint of the segment on line M
. Denote by θthe angle made by the line segment and the ray L at the point where they meet.
For the position of the line segment making b maximal, express a, b and θ in terms of φ. Show your
work. You may ﬁnd the law of cosines useful,
c2
= a2
+ b2
−2ab cos(φ).
Solution to Problem 2 We start with the law of cosines:
c2
= a2
+ b2
−2ab cos φ

We can consider b as a function of a : in fact, there are two such functions, however, their maximums
are the same and they take on their maximum at the same point.
Implicitly diﬀerentiating (4) with respect to a, we obtain
At the maximum of b= b(a), we know that db = 0. Therefore
Plugging this back into (4), we obtain
therefore the maximum b is

which is assumed for
We’re also required to find an expression for θ when b is maximum. We call α the angle
corresponding to the edge b in the abc triangle, then by sines theorem
But this, combined with (5), implies that sin α = 1, and hence α = π/2. From the relation
φ + θ + α = π, we ﬁnd out that
in the case when b is maximal
Problem 3 Solve Problem 17 from §4.4, p. 137 of the textbook. You are free to use any (valid)
method you like. You may find the following remarks useful.
Figure 6 depicts an isosceles triangle circumscribed about a circle of radius R. The two similar sides
each have length A + B, and the third side has length 2A. Express the area of each right triangle in
terms of either tan(α) or tan(β). Because the sum of the angles of a circle is 2π, β equals π − 2α.
Recall the doubleangle formula for tangents,

and the complementary angle formula for tangents,
Figure 6: An isosceles triangle circumscribing a circle of radius R

Using these, express tan(β), and thus the total area of the triangle, in terms of T = tan(α). Now
minimize this expression with respect to T , ﬁnd the corresponding angle α, and the height of the
triangle.
Solution to Problem 3 The total area can be written down as
where u = tan α. In order to find the extrema of this last quantity, all we need to do is to differentiate
with respect to u and set the result equal to zero:

hence
therefore u2 = 0 or u2 = 3. Recalling that u = tan α, which needs to be a positive number for the
area to be maximum, we conclude
hence

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