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chapter7.2r
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à 7.2èThe Circle
äïPlease use the distance formula to find the distance
êêbetween two points.
âè Find the distance between the points, (-1, 4) and (3, 1).
#êë ┌─────────────────────ë┌──────────────────────
#êdï=ïá(x½ - x¬) + (y½ - y½)ï=ïá(3 - (-1))ì + (1 - 4)ì
#êêêè ┌──────ë┌───
#êêê =ïá16 + 9ï=ïá 25ï=ï5
#éSêêêêïGiven two points, (x¬, y¬)
#êêêêëand (x½, y½), it is possible to
êêêêëcomplete a right triangle with ç
êêêêëtwo points as vertices.ïThe third
#êêêêëvertex must be (x½, y¬).ïBy sub-
êêêêëtracting coordinates, the lengths of
@fig7201.bmp,15,25
#the horizontal & vertical sides are found to be (x½ - x¬) and (y½ - y¬).
The third side, d, is then found using the Pythagorean formula.
#ë ┌───────────────────────ë This formula can be used to find
#dï=ïá(x½ - x¬)ì + (y½ - y¬)ìë the distance between any two points.
In the example, the coordinates of the two points, (-1, 4) and (3, 1),
are substituted into this formula and simplified.
#ë ┌──────────────────────ë┌──────ë┌───êïThus, the
#dï=ïá(3 - (-1))ì + (1 - 4)ìï=ïá16 + 9ï=ïá 25ï=ï5èdistance is 5
1êïFind the distance between the points,
êêêë(4, -6) and (-2, 2).
#êêêêêêë ┌─
#ë A)ï14êëB)ï10êëC)ï4∙á3êèD)ïå
ü
#êë┌───────────────────────ë┌────────ë┌────
#ë dï=ïá(4 - (-2))ì + (-6 - 2)ìï=ïá 36 + 64ï=ïá 100ï=ï10
Ç B
2êïFind the distance between the points,
êêêë(-5, 14) and (4, 2).
#êë┌─êêï┌─
#ë A)ï9∙á5êèB)ï5∙á2êèC)ï15êëD)ïå
ü
#êë┌───────────────────────ë┌─────────ë┌────
#ë dï=ïá(4 - (-5))ì + (2 - 14)ìï=ïá 81 + 144ï=ïá 225ï=ï15
Ç C
3êïFind the distance between the points,
êêêë(-3, -1) and (4, 6).
#êë┌─êêêêë ┌─
#ë A)ï7∙á2êèB)ï6êë C)ï9∙á3êèD)ïå
ü
#êë┌────────────────────────ë┌────────ë┌───ê┌─
#ë dï=ïá(4 - (-3))ì + (6 - (-1)ìï=ïá 49 + 49ï=ïá 98ï=ï7∙á2
Ç A
äïPlease use the distance formula to find the equation of
êêthe circle with given center and radius.
âëFind the equation of the circle with center, (3, 2),
êëand radius 5.
#êêê (x - h)ì + (y - k)ìï=ïrì
#êêê (x - 3)ì + (y - 2)ìï=ï25
êèThis is the equation of the circle in standard form.
éSïTo find the equation of the circle with radius, r, and center,
(h, k), you should first let (x, y) be an arbitrary point on the circle.
Since the circle is the set of all points that are a distance, r, from
the center, (h, k), you can use the distance formula to obtain an
equation of the circle.
#êêêê ┌───────────────────
#êêêïrï=ïá(x - h)ì + (y - k)ì
#(Square both sides.)è(x - h)ì + (y - k)ìï=ïrì
This formula gives you the equation of the circle in standard form.
In the example, the radius, 5, and the coordinates of the center, (3, 2),
are substituted into this formula for r, h, and k.
#êêêï(x - 3)ì + (y - 2)ìï=ï25
This is the equation in standard form of the circle with center, (3, 2),
and radius, 5.
4èFind the equation of the circle with radius, 2, and
êëcenter (-1, 5).
#ëA)ï(x - 2)ì + (y + 1)ìï=ï25ëB)ï(x - 5)ì + (y - 1)ì =ï4
#ëC)ï(x + 1)ì + (y - 5)ìï=ï4ë D)ïå
ü
#êêêï(x - h)ì + (y - k)ìï=ïrì
#êêêï(x + 1)ì + (y - 5)ìï=ï4
Ç C
5èFind the equation of the circle with radius, 3, and
êëcenter (-4, -5).
#ëA)ï(x - 4)ì + (y - 3)ìï=ï25ëB)ï(x + 4)ì + (y + 5)ì =ï9
#ëC)ï(x - 4)ì + (y - 5)ìï=ï3ë D)ïå
ü
#êêêï(x - h)ì + (y - k)ìï=ïrì
#êêêï(x + 4)ì + (y + 5)ìï=ï9
Ç B
6èFind the equation of the circle with radius, 7, and
êëcenter (0, 0).
#ëA)ïxì + yìï=ï49êêèB)ï(x + 1)ì + (y + 1)ì =ï49
#ëC)ï(x - 1)ì + (y - 1)ìï=ï49ëD)ïå
ü
#êêêï(x - h)ì + (y - k)ìï=ïrì
#êêêï(x - 0)ì + (y - 0)ìï=ï49
#êêêêë xì + yìï=ï49
Ç A
äïPlease complete the square on both variables to find the
êêcenter and radius of the circles.
âê Find the center and radius of the circle,
#êêê xì + yì - 2x + 4y + 1ï=ï0.
#êêè xì - 2xë+ yì + 4yë =ï-1
#êêè xì - 2x + 1 + yì + 4y + 4ï=ï-1 + 1 + 4
#êêêè(x - 1)ì + (y + 2)ìï=ï4
êê The center is (1, -2), and the radius is 2.
éSèTo find the center and radius of the circle,
#êêê xì + yì - 2x + 4y + 1ï=ï0,
you should group the x-terms together, group the y-terms together, and
move the constant term to the right side of the equation.
#êêè xì - 2xë+ yì + 4yë =ï-1
To complete the square on x, you should multiply "1/2" times the coeffi-
cient of x and square the result.ïThis number is added to both sides
of the equation and grouped with the x-terms.ïSimilarly, you should
multiply "1/2" times the coefficient of y, square the result, and add
to both sides.
#êêè(xì - 2x + 1) + (yì + 4y + 4)ï=ï-1 + 1 + 4
Now, you should factor the two trinomials and combine terms on the
#right side.êë(x - 1)ì + (y + 2)ìï=ï4
The center is seen to be, (1, -2), and the radius is 2.ïNote, if the
number on the right comes out to be zero, then the graph is just the
center point.ïAlso, if the number on the right comes out to be
negative, then there is no graph.
7ïIf possible, find the center and radius of the equation,
#êêêxì + yì + 4x - 10y + 20ï=ï0.
êè A)ï(-3, 6), r = 4êèB)ïGraph is the point (-2, 5)
êè C)ï(-2, 5), r = 3êèD)ïå
ü
#êêêxì + yì + 4x - 10y + 20ï=ï0
#êêèxì + 4xë+ yì - 10yê=ï-20
#êêèxì + 4x + 4 + yì - 10y + 25ï= -20 + 4 + 25
#êêêè (x + 2)ì + (y - 5)ìï=ï9
êê The center is (-2, 5), and the radius is 3.
Ç C
8ïIf possible, find the center and radius of the equation,
#êêêxì + yì - 6x - 8y + 9ï=ï0.
êè A)ï(3, 4), r = 4êè B)ï(-3, -4), r = 3
êè C)ïNo graphêêïD)ïå
ü
#êêê xì + yì - 6x - 8y + 9ï=ï0
#êêèxì - 6xë+ yì - 8yê=ï-9
#êêèxì - 6x + 9 + yì - 8y + 16ï= -9 + 9 + 16
#êêêè(x - 3)ì + (y - 4)ìï=ï16
êê The center is (3, 4), and the radius is 4.
Ç A
9ïIf possible, find the center and radius of the equation,
#êêêxì + yì + 10x - 12y + 61ï=ï0.
êè A)ïNo graphêêïB)ïGraph is the point (-5, 6)
êè C)ï(5, -6), r = 1êèD)ïå
ü
#êêê xì + yì + 10x - 12y + 61ï=ï0
#êêèxì + 10xë+ yì - 12yê=ï-61
#êêèxì + 10x + 25 + yì - 12y + 36ï= -61 + 25 + 36
#êêêè(x + 5)ì + (y - 6)ìï=ï0
êêë The graph is the point (-5, 6).
Ç B
äïPlease graph the following circles.
#âêGraph the circle, (x - 1)ì + (y + 2)ìï=ï4.
êêêèPlease see Details.
éS
To graph the circle,
#(x - 1)ì + (y + 2)ìï=ï4, you should
locate the center, (1, -2), on the
coordinate system and draw a new coordinate
system through the center.ïThen, you should
locate the intercepts relative to the new
coordinate system by counting over two units
in each direction.ïThen join the intercepts
with a smooth curve.
@fig7210.bmp,375,100
# 10êêGraph, xì + yìï=ï4.
@fig7204.bmp,28,229
@fig7205.bmp,234,229
@fig7206.bmp,450,229
#üêêêïGraph, xì + yìï=ï4.
@fig7202.bmp,50,450
Ç B
# 11êèGraph, (x - 2)ì + (y - 3)ìï=ï1.
@fig7207.bmp,25,229
@fig7208.bmp,250,229
@fig7209.bmp,450,229
#üêêè Graph, (x - 2)ì + (y - 3)ìï=ï1.
@fig7203.bmp,50,450
Ç C