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à 7.1ïThe Parabola
äïPlease find the x-intercepts of the following parabolas.
#âïFind the x-intercepts of the parabola, y = xì - 6x + 8.
#êêè xì - 6x + 8ï=ï0
êêï(x - 2)(x - 4)ï=ï0êThe x-intercepts are the
#êêx - 2 = 0ï│ïx - 4 = 0ë points, (2, 0) and (4, 0).
#êêè x = 2ï│ïx = 4
#éSAll equations of the form, y = axì + bx + c, have graphs that are
#parabolas,aƒ0.ïIn Section 7.5, we looked at graphing ç functions by
plotting points.ïIn this section, we are going to look at finding the
intercepts, the axis of symmetry, and the vertex of each of ç
curves.ïVery often finding this additional information reduces the num-
ber of points required to get a graph.ïFirst, we will find the x-inter-
cepts.ïThese are the points where the curve crosses the x-axis.ïThey
are found by setting the function equal to zero and solving for x.
#êêïxì - 6x + 8ï=ï0è ┌─¥ïx - 2 = 0ï│ïx - 4 = 0
#êê(x - 2)(x - 4)ï=ï0 ──┘ê x = 2ï│ïx = 4
#The x-intercepts of the parabola, y = xì - 6x + 8, are (2, 0) and (4,0).
1êïFind the x-intercepts of the parabola,
#êêêêïyï=ïxì - 4.
êèA)ï(-4, 0), (4, 0)êë B)ï(2, 0), (-2, 0)
êèC)ï(1, 0), (0, 4)êêD)ïå
#üêêêèxì - 4ï=ï0
êêêë(x - 2)(x + 2)ï=ï0
#êêêèx - 2 = 0ï│ïx + 2 = 0
#êêêêx = 2ï│ïx = -2
êêïThe x-intercepts are (2, 0) and (-2, 0).
Ç B
2êïFind the x-intercepts of the parabola,
#êêêëyï=ï-xì + 2x + 3.
êèA)ï(0, -3), (0, 1)êë B)ï(1, 0), (-3, 0)
êèC)ï(-1, 0), (3, 0)êë D)ïå
#üêêêxì - 2x - 3ï=ï0
êêêë(x + 1)(x - 3)ï=ï0
#êêêèx + 1 = 0ï│ïx - 3 = 0
#êêêë x = -1ï│ïx = 3
êêïThe x-intercepts are (-1, 0) and (3, 0).
Ç C
3êïFind the x-intercepts of the parabola,
#êêêëyï=ï6xì + x - 2.
êèA)ï(1/2, 0), (-2/3, 0)êïB)ï(-2, 0), (6, 0)
êèC)ï(3, 0), (-6, 0)êë D)ïå
#üêêê6xì + x - 2ï=ï0
êêêè(2x - 1)(3x + 2)ï=ï0
#êêêï2x - 1 = 0ï│ï3x + 2 = 0
#êêêëx = 1/2ï│ïx = -2/3
êêïThe x-intercepts are (1/2, 0) and (-2/3, 0).
Ç A
4êïFind the x-intercepts of the parabola,
#êêêëyï=ïxì - 2x - 1.
#êêêêêêêï┌─êè┌─
#êèA)ï(-2, 0), (1, 0)êë B)ï(1 + á2, 0), 1 - á2, 0)
êèC)ï(2, 0), (-1, 0)êë D)ïå
#üêêêxì - 2x - 1ï=ï0
#êêë ┌────────────────ê┌─êï┌─
#êë-(-2) ± á(-2)ì - 4∙1∙(-1)è2 ± á8è2 ± 2∙á2ê┌─
#ê x = ───────────────────────── = ────── = ──────── = 1 ± á2
êêê 2∙1êêè2êè2
#êêêêêê┌─êê┌─
#êêïThe x-intercepts are (1 + á2, 0) and (1 - á2, 0).
Ç B
5êïFind the x-intercepts of the parabola,
#êêêëyï=ïxì - 6x + 9.
êèA)ï(1, 0), (-9, 0)êë B)ï(3, 0), (-3, 0)
êèC)ï(3, 0)êêê D)ïå
#üêêê xì - 6x + 9ï=ï0
êêêè(x - 3)(x - 3)ï=ï0
#êêêèx - 3 = 0ï│ïx - 3 = 0
#êêêêx = 3ï│ïx = 3
êêêïThe x-intercept is (3, 0).
Ç C
6êïFind the x-intercepts of the parabola,
#êêêëyï=ïxì - 2x + 2.
êèA)ï(-1, 0), (-2, 0)êëB)ï(1, 0), (2, 0)
êèC)ï(-1, 0), (2, 0)êë D)ïå
#üêêêxì - 2x + 2ï=ï0
#êêë ┌────────────────ê┌──
#êë-(-2) ± á(-2)ì - 4∙1∙(2)è 2 ± á-4è2 ± 2∙i
#ê x = ───────────────────────── = ─────── = ─────── = 1 ± i
êêê 2∙1êêè 2êï2
Since ç are complex solutions, this parabola does not cross the
x-axis, and there are no x-intercepts.
Ç D
äïPlease find the y-intercept of each of the following
êêparabolas.
âêè Find the y-intercept of the parabola,
#êêêë y = xì - 6x + 4.
#êïLet x equal 0.è y = 0ì - 6∙0 + 4
êêêë y = 4
êêê The y-intercept is (0, 4).
éSëThe y-intercept is found by letting x equal zero and solving
#êëfor y.êèy = xì - 6x + 4
#êêêêy = 0ì - 6∙0 + 4
êêêêy = 4
êêêïThe y-intercept is (0, 4).
7êïFind the y-intercept of the parabola,
#êêêê yï=ïxì - 4.
êèA)ï(2, 0), (0, 2)êêB)ï(4, 0)
êèC)ï(0, -4)êêêD)ïå
#üêêêïyï=ïxì - 4
#êêêê yï=ï0ì - 4
êêêê yï=ï-4
êêêïThe y-intercept is (0, -4).
Ç C
8êïFind the y-intercept of the parabola,
#êêêëyï=ï-xì + 2x + 3.
êèA)ï(0, 3)êêê B)ï(0, -3)
êèC)ï(1, 3)êêê D)ïå
#üêêë yï=ï-xì + 2x + 3
#êêêëyï=ï-0ì + 2∙0 + 3
êêêëyï=ï3
êêêïThe y-intercept is (0, 3).
Ç A
9êïFind the y-intercept of the parabola,
#êêêëyï=ï6xì + x - 2.
êèA)ï(0, -2)êêêB)ï(0, -3)
êèC)ï(2, 0)êêê D)ïå
#üêêë yï=ï6xì + x - 2
#êêêëyï=ï6∙0ì + 0 - 2
êêêëyï=ï-2
êêêïThe y-intercept is (0, -2).
Ç A
10ê Find the y-intercept of the parabola,
#êêêëyï=ïxì - 2x - 1.
êèA)ï(-1, 0)êêêB)ï(0, -1)
êèC)ï(0, 1)êêê D)ïå
#üêêë yï=ïxì - 2x - 1
#êêêëyï=ï0ì - 2∙0 - 1
êêêëyï=ï-1
êêêïThe y-intercept is (0, -1).
Ç B
11ê Find the y-intercept of the parabola,
#êêêëyï=ïxì - 6x + 9.
êèA)ï(0, 9)êêê B)ï(0, 3)
êèC)ï(0, -3)êêêD)ïå
#üêêë yï=ïxì - 6x + 9
#êêêëyï=ï0ì - 6∙0 + 9
êêêëyï=ï9
êêê The y-intercept is (0, 9).
Ç A
12ê Find the y-intercept of the parabola,
#êêêëyï=ïxì - 2x + 2.
êèA)ï(0, -2)êêêB)ï(-2, 0)
êèC)ï(0, 2)êêê D)ïå
#üêêë yï=ïxì - 2x + 2
#êêêëyï=ï0ì - 2∙0 + 2
êêêëyï=ï2
êêê The y-intercept is (0, 2).
Ç C
äèPlease find the axis of symmetry for the following
êê parabolas.
#âè Find the axis of symmetry for, y = xì - 6x + 8.
êêêêbê -6ë6
#êêëxï=ï- ──ï=ï- ───ï=ï─ï=ï3
êêêë 2aê2∙1ë2
êè The axis of symmetry is the vertical line, x = 3.
#éSèThe axis of symmetry of the parabola, y = axì + bx + c, is the
vertical line passing through the middle of the parabola exactly half
way between the x-intercepts.ïThe x-intercepts are given by the quadra-
#tic formula.êè┌────────êê┌────────
#êêè-b + ábì - 4acêï-b - ábì - 4ac
#êêè──────────────èandè──────────────
êêê 2aêêê 2a
If ç two intercepts are added and divided by 2, the result is -b/2a.
This gives us a formula for the axis of symmetey,ïx = -b/2a.
#In the example, the axis of symmetry for, y = xì - 6x + 8, is given by
êêêê bê-(-6)ë6
#êêë xï=ï- ──ï=ï- ─────ï=ï─ï=ï3.
êêêê2aê 2∙1ë 2
The axis of symmetry is helpful in drawing graphs of parabolas.ïAny
time you have a point on one side, there must be a corresponding point
on the other side.
13è Find the axis of symmetry for the parabola,
#êêêê yï=ïxì - 4.
êèA)ïx = 0êêêïB)ïx = -4
êèC)ïx = 4êêêïD)ïå
#üêêêïyï=ïxì - 4
êêêêïbê0
#êêêxï=ï- ── =ï- ───ï=ï0
êêêê 2aë 2∙1
êêë The axis of symmetry is x = 0.
Ç A
14ê Find the axis of symmetry of the parabola,
#êêêêyï=ï-xì + 2x + 3.
êèA)ïx = -2êêê B)ïx = -3
êèC)ïx = 1êêêïD)ïå
#üêêë yï=ï-xì + 2x + 3
êêêêèbêï2
#êêê xï=ï- ──ï=ï- ─────ï=ï1
êêêêï2aê2(-1)
êêêïThe axis of symmetry is x = 1.
Ç C
15ê Find the axis of symmetry for the parabola,
#êêêëyï=ï6xì + x - 2.
êèA)ïx = 1êêêïB)ïx = -1/12
êèC)ïx = -2êêê D)ïå
#üêêë yï=ï6xì + x - 2
êêêêèbê 1êï1
#êêê xï=ï- ──ï=ï- ───ï=ï- ──
êêêêï2aê2∙6ê12
êêêïThe axis of symmetry is x = -1/12.
Ç B
16ê Find the axis of symmetry for the parabola,
#êêêëyï=ïxì - 2x - 1.
êèA)ïx = 2êêêïB)ïx = -2
êèC)ïx = 1êêêïD)ïå
#üêêë yï=ïxì - 2x - 1
êêêêïbê -2
#êêêxï=ï- ──ï=ï- ───ï=ï1
êêêê 2aê2∙1
êêë The axis of symmetry is x = 1.
Ç C
17ê Find the axis of symmetry for the parabola,
#êêêëyï=ïxì - 6x + 9.
êèA)ïx = 6êêêïB)ïx = 3
êèC)ïx = -3êêê D)ïå
#üêêë yï=ïxì - 6x + 9
êêêêïbê -6
#êêêxï=ï- ──ï=ï- ───ï=ï3
êêêê 2aê2∙1
êêê The axis of symmetry is x = 3.
Ç B
18ê Find the axis of symmetry for the parabola,
#êêêëyï=ïxì - 2x + 2.
êèA)ïx = 1êêêïB)ïx = -3/4
êèC)ïx = 3/2êêêD)ïå
#üêêë yï=ïxì - 2x + 2
êêêêïbê -2
#êêêxï=ï- ──ï=ï- ───ï=ï1
êêêê 2aê2∙1
êêê The axis of symmetry is x = 1.
Ç A
äïPlease find the vertex of each of the following parabolas.
#âêè Find the vertex of y = xì - 6x + 8.
êêêêêëbê-6
# vertexï=ï(-b/2a, f(-b/2a)),è1)ï- ──ï=ï- ───ï=ï3
êêêêêè 2aê2∙1
#êêêêë2)ïf(3)ï=ï3ì - 6∙3 + 8ï=ï-1
êêêè The vertex is (3, -1).
éSïThe vertex of a parabola is the lowest (or highest) point on
the curve.ïSince the axis of symmetry passes through the vertex, the
first coordinate of the vertex must be, -b/2a.ïThe second coordinate
is f(-b/2a).
êêëVertexï=ï(-b/2a, f(-b/2a))
#In the example, y = xì - 6x + 8, the first coordinate of the vertex is
-b/2a = 3.ïThe second coordinate is f(3) or y evaluated at 3.
#êêêêyï=ï3ì - 6∙3 + 8
êêêêè yï=ï-1
#Thus, the vertex of, y = xì -6x + 8, is (3, -1).
# 19êèFind the vertex of, y = xì - 4.
êêA)ï(0, -4)êêè B)ï(-2, 4)
êêC)ï(2, 4)êêëD)ïå
#üêêêèyï=ïxì - 4
êêFirst coordinateêïSecond coordinate
êê bê 0
#êë- ──ï=ï- ───ï=ï0,ë yï=ï0ì - 4ï=ï-4
êê2aê2∙1
êêêèThe vertex is (0, -4).
Ç A
# 20ê Find the vertex of, y = -xì + 2x + 3.
êêA)ï(2, -3)êêè B)ï(-1, 5)
êêC)ï(1, 4)êêëD)ïå
#üêêêyï=ï-xì + 2x + 3
êêFirst coordinateêëSecond coordinate
êê bêï2
#êë- ──ï=ï- ──────ï=ï1,ë yï=ï-1ì + 2∙1 + 3ï=ï4
êê2aê2∙(-1)
êêêèThe vertex is (1, 4).
Ç C
# 21ê Find the vertex of, y = 6xì + x - 2.
êêA)ï(-1/2, -49/24)êè B)ï(-1, 4)
êêC)ï(3, 2)êêëD)ïå
#üêêêyï=ï6xì + x - 2
êè First coordinateêê Second coordinate
êëbêï1êè 1êê 49
#êï- ──ï=ï- ──────ï=ï- ──,ë yï=ï- ──
êè 2aê2∙(6)ê 12êê 24
êêêThe vertex is (-1/12, -49/24).
Ç A
#F 22êê Graph, yï=ïxì - 4.
A) 1,0,4
B) 1,0,-4
C) 1,-4,4
D) 1,4,4
#üè Graph, yï=ïxì - 4
x-intercepts (2,0),(-2,0)
y-intercept (0,-4)
axis of symmetry x = 0
vertexï(0,-4)
Ç B
#F 23êëGraph, yï=ï-xì + 2x + 3.
A) 1,-2,3
B) 1,0,3
C) -1,2,3
D) 1,0,-3
#üè Graph, yï=ï-xì + 2x + 3
x-intercepts (-1,0),(3,0)
y-intercept (0,3)
axis of symmetry x = 1
vertexï(1,4)
Ç C
#F 24êëGraph, yï=ï6xì + x - 2.
A) 6,1,-2
B) -6,-1,2
C) 1,-2,0
D) -1,2,0
#üè Graph, yï=ï6xì + x - 2
x-intercepts (1/2,0),(-2/3,0)
y-intercept (0,-2)
axis of symmetry x = -1/12
vertexï(-1/12, -49/24)
Ç A
#F 25êëGraph, yï=ïxì - 2x - 1.
A) -1,2,1
B) 1,0,0
C) 1,-2,-1
D) -1,4,-4
#üè Graph, yï=ïxì - 2x - 1
#êêï┌─ê┌─
#x-intercepts (1+á2,0),(1-á2,0)
y-intercept (0,-1)
axis of symmetry x = 1
vertexï(1, -2)
Ç C