Name
ID
MATH 251
1-12
/60
Fall 2012
13
/10
P. Yasskin
14
/10
15
/10
16
/10
Total
/100
Exam 1
Sections 515
Solutions
Multiple Choice: (5 points each. No part credit.)
1. Find the area of the triangle whose vertices are
2, 4, 3 , Q
P
and R
3, 4, 2
0, 6, 3 .
a. 1
Correct Choice
3
b.
c. 2 3
d. 6
e. 12
SOLUTION:
PQ
A
PR
1 PQ
2
PQ
Q
î
k
1
0 1
P
1, 0, 1
î0
2
2 2 0
1 4
PR
2
4
4
PR
0
2
R
k2
P
2, 2, 0
0
2, 2, 2
3
2. Which of the following is the plane which passes through the point
the line
x, y, z
1
a. 2x
y
3z
13
b. 2x
y
3z
9
c. 2x
y
3z
13
d. 2x
y
3z
9
e.
2x
y
3z
2t, 2
t, 3
4, 2, 1
and is perpendicular to
3t ?
Correct Choice
9
SOLUTION:
The direction of the line is v
2, 1, 3 , which must be the normal N
the plane. Since it contains the point P
4, 2, 1 , the plane is N X N P or
2x y 3z 2 4 1 2 3 1 13.
2, 1, 3
to
1
3. The quadratic surface x 2
a.
b.
c.
d.
e.
y2
an elliptic hyperboloid
a hyperbolic paraboloid
a hyperboloid of 1 sheet
a hyperboloid of 2 sheets
a cone.
z2
4x
4y
6z
0 is
Correct Choice
SOLUTION:
We complete the squares to get
2
x 2
y 2 2 z 3 2 4 4 9
1
Since there are squares on each variable with different signs and non-zero on the right, it is a
hyperboloid. Rearranging, we have
z 3 2 1 x 2 2 y 2 2
So z 3 2 can never be zero and it must be a hyperboloid of 2 sheets.
4.
The plot at the right is the graph of which equation?
x2
a. z
b. z
y2
x2
c. z 2
e. z 2
y2
x2
d. z 2
Correct Choice
y2
x2
x2
SOLUTION:
y2
y2
1
The saddle surface is a hyperbolic paraboloid: (a) or (b).
Since it goes down in the x-direction and up in the y-direction, it is (a).
5. An airplane is travelling due South with constant speed and constant altitude as it flies over College
Station. Since its path is part of a circle around the earth, its acceleration points directly toward the
center of the earth. In which direction does it binormal B point?
a. Up
b. North
c. East
Correct Choice
d. South
e. West
SOLUTION:
v is South. a is Down.
v
â
So B
points East by the right hand rule.
|v â|
2
6. For the curve r t
4 cos t,
a. v
4 sin t,
3,
b. a
4 cos t,
0,
4 cos t
4 sin t
d. Arc length between t
e. a T
which of the following is FALSE?
4 sin t
Correct Choice
25
c. |v |
3t,
0 and t
is 10
2
0
SOLUTION:
v and a are correct by differentiation.
|v |
9
16 sin 2 t
L
ds
|v | dt
16 cos 2 t
5
2
5 dt
aT
2
0
5t
0
d|v |
dt
0
10
7. A wire in the shape of the curve r t
4 cos t, 3t,
0 and t 2 .
Find its total mass between t
has linear mass density
4 sin t
y
z.
a. 6
b. 12
c. 30
2
d. 6
e. 30
2
Correct Choice
SOLUTION:
2
M
ds
y
z |v | dt
3t
15t 2
2
4 sin t 5 dt
0
8. Find the work done to move an object along the curve r t
between t
a.
32
b.
25
c.
25
0 and t
2
by the force F
z, 0, x
4 cos t,
2
20 cos t
30
2
0
3t,
4 sin t
?
Correct Choice
2
d. 25
e. 32
SOLUTION:
Frt
4 sin t,
2
W
F ds
2
Frt
0
0,
v dt
0
4 cos t
16 sin 2 t
v
16 cos 2 t dt
4 sin t,
3,
4 cos t
2
16 dt
32
0
3
xe y at the point
9. Find the plane tangent to the graph of z
2, 0 . Its z-intercept is
a. e
b. 2
Correct Choice
c. 0
d.
2
e.
e
SOLUTION:
f
xe y
f 2, 0
fx
ey
f x 2, 0
1
fy
xe y
f y 2, 0
2
2
z
f 2, 0
2
f x 2, 0 x
1x
When x
10. Find the plane tangent to the graph of xz 3
2
2
zy 2
0
2y
0, we have z
y
f y 2, 0 y
yx 4
2
2
42 at the point
0.
1, 2, 0 . Its z-intercept is
a. 10
b. 5
4
c. 5
2
d. 2
5
e. 4
5
Correct Choice
xz 3 zy 2 yx 4
N X N P
SOLUTION:
N
F
F x, y, z
8, 1, 4
When x
0, we have z
1,2,0
y
F
8x
z 3 4yx 3 , 2zy x 4 , 3xz 2 y 2
y 4z 8 1 2 4 0 10
5.
2
4
11. Hans Duo is currently at x, y, z
field whose density is
as fast as possible?
a.
6, 1, 13
b.
1
206
c.
6, 1, 13
d.
1
206
e.
1
6, 1, 13
206
x z
3, 2, 1 and flying the Milenium Eagle through a deadly polaron
yz . In what unit vector direction should he travel to reduce the density
2
6, 1, 13
6, 1, 13
36
Correct Choice
2xz, z 2 , x 2
SOLUTION:
|v |
2
1
169
2yz
206
12. The point
v
v
v
|v |
3,2,1
1
206
6, 1, 13
6, 1, 13
x, y
9, 3 is a critical point of the function f x, y
Derivative Test to classify this critical point.
x2
2xy 2
4y 3 . Use the Second
a. local minimum
b. local maximum
c. saddle point
Correct Choice
d. TEST FAILS
SOLUTION:
f x 2x 2y 2
f x 9, 3
18 18 0
Checked
2
fy
4xy 12y
f y 9, 3
4 27 12 9 0
Checked
f xx 2
f xx 9, 3
2
f yy
4x 24y
f yy 9, 3
36 72 36
f xy
4y
f xy 9, 3
12
2
2
D f xx f yy f xy
2 36 12
72
Since D 0 it is a saddle point.
5
Work Out: (10 points each. Part credit possible. Show all work.)
13. Find the scalar and vector projections of the vector a
SOLUTION:
a b
b
comp b a
proj b a
a b
2
2
4
4
b b
4
along the vector b
1, 2, 2
1
4
9
b
2, 1, 2 .
3
4
3
a bb
b b
4 2, 1, 2
9
8, 4, 8
9 9 9
14. The pressure, P, volume, V, and temperature, T, of an ideal gas are related by
P
kT
V
3
cm -atm
.
°K
2000 cm 3 , and T 300 °K,
for some constant k. For a certain sample k
10
At a certain instant, the volume and temperature are V
3
cm
°K
and are increasing at dV
40
, and dT
5
.
dt
dt
sec
sec
At that instant, what is the pressure, is it increasing or decreasing and at what rate?
SOLUTION:
dP
dt
P dV
V dt
P
kT
V
P dT
T dt
3
10 300 cm -atm °K
3
2000
°K
cm
kT dV
V 2 dt
k dT
V dt
1. 5 atm
10 300
2000 2
40
10 5
2000
5
1000
0. 005
atm
sec
Since dP is negative, the pressure is decreasing.
dt
6
15. If two resistors, with resistances R 1
and R 2 , are arranged in parallel, the total resistance R is
given by
R1R2
1
1
or
R
R1
R2
R1 R2
If R 1 4
and R 2 6
and the uncertainty in the measurement of R 1 is
R 1 0. 03
R 2 0. 02 . Find R and use differentials to estimate the uncertainty in the
for R 2 is
measurment of R.
1
R
SOLUTION:
R
R R
1
R1
R2 2
R1 R2
R
2
4 6
4 6
R R
2
R2
R1
2. 4 .
R1 R2 R2 R1R2 1
R1
R1 R2 2
R1 2
62
R2
. 03
2
R1 R2
4 6 2
R1
42
4 6
R2 R1 R1R2 1
R1 R2 2
2
. 02
and
R2
1. 08 0. 32
100
0. 014
16. Find the point(s) on the surface z 2
46 2x 4y which are closest to the origin.
HINT: Explain why you can minimize the square of the distance instead of the distance.
Use the Second Derivative Test to check it is a local minimum.
SOLUTION: We need to minimize the distance from the point
x, y, 46 2x 4y
to the origin.
We can minimize the square of the distance because as the distance decreases, so does it’s square.
So we minimize f x 2 y 2 z 2 x 2 y 2 46 2x 4y
f x 2x 2 0
x 1
f y 2y 4 0
y 2
z
46 2x 4y
36
6
So the points are 1, 2, 6 and 1, 2, 6
f xx 2 0
f yy 2
f xy 0
D f xx f yy f xy 2 4 0
local minimum
7