James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

Chapter 10
Concept Check Answers
Cut here and keep for reference
1. (a) What is a parametric curve?
A parametric curve is a set of points of the form
sx, yd − s f std, tstdd, where f and t are functions of a variable
t, the parameter.
(b) How do you sketch a parametric curve?
Sketching a parametric curve, like sketching the graph of
a function, is difficult to do in general. We can plot points
on the curve by finding f std and tstd for various values
of t, either by hand or with a calculator or computer.
Sometimes, when f and t are given by formulas, we can
eliminate t from the equations x − f std and y − tstd to get
a Cartesian equation relating x and y. It may be easier to
graph that equation than to work with the original formulas
for x and y in terms of t.
2. (a) How do you find the slope of a tangent to a parametric
curve?
You can find dyydx as a function of t by calculating
dy
dx − dyydt if dxydt ± 0
dxydt
(b) How do you find the area under a parametric curve?
If the curve is traced out once by the parametric equations
x − f std, y − tstd, < t < , then the area is
A − y b
y dx − y tstd f 9std dt
a
for y
tstd f 9std dt if the leftmost point is s f sd, tsdd
rather than s f sd, tsddg.
3. Write an expression for each of the following:
(a) The length of a parametric curve
If the curve is traced out once by the parametric equations
x − f std, y − tstd, < t < , then the length is
L − y ssdxydtd2 1 sdyydtd 2 dt
− y sf f 9stdg2 1 ft9stdg 2 dt
(b) The area of the surface obtained by rotating a parametric
curve about the x-axis
S − y 2yssdxydtd 2 1 sdyydtd 2 dt
− y 2tstd sf f 9stdg 2 1 ft9stdg 2 dt
4. (a) Use a diagram to explain the meaning of the polar coordinates
sr, d of a point.
y
P(r, ¨)=P(x, y)
O
¨
r
x
y
x
(b) Write equations that express the Cartesian coordinates
sx, yd of a point in terms of the polar coordinates.
x − r cos
y − r sin
(c) What equations would you use to find the polar coordinates
of a point if you knew the Cartesian coordinates?
To find a polar representation sr, d with r > 0 and
0 < , 2, first calculate r − sx 2 1 y 2 . Then is
specified by tan − yyx. Be sure to choose so that sr, d
lies in the correct quadrant.
5. (a) How do you find the slope of a tangent line to a polar
curve?
dy d
dy
dx − d d syd
− −
dx d
d d sxd
S dr sin 1 r cos
dD
−
S
dD dr cos 2 r sin
d
sr sin d
d
d
sr cos d
d
where r − f sd
(b) How do you find the area of a region bounded by a polar
curve?
A − y b 1
2 r 2 d − y b 1
2 f f sdg2 d
a
(c) How do you find the length of a polar curve?
L − y b
a ssdxydd2 1 sdyydd 2 d
− y b
sr 2 1 sdrydd 2 d
a
− y b
sf f sdg2 1 f f 9sdg 2 d
a
6. (a) Give a geometric definition of a parabola.
A parabola is a set of points in a plane whose distances
from a fixed point F (the focus) and a fixed line l (the
directrix) are equal.
(b) Write an equation of a parabola with focus s0, pd and
directrix y − 2p. What if the focus is sp, 0d and the
directrix is x − 2p?
In the first case an equation is x 2 − 4py and in the second
case, y 2 − 4px.
7. (a) Give a definition of an ellipse in terms of foci.
An ellipse is a set of points in a plane the sum of whose
distances from two fixed points (the foci) is a constant.
(b) Write an equation for the ellipse with foci s6c, 0d and
vertices s6a, 0d.
x 2
a 1 y 2
2 b − 1 2
where a > b . 0 and c 2 − a 2 2 b 2 .
a
(continued)
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