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10.5 Graphing quadratic equations  (Page 6/15)

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The quadratic equation h = −16 t 2 + 128 t + 32 is used to find the height of a stone thrown upward from a height of 32 feet at a rate of 128 ft/sec. How long will it take for the stone to reach its maximum height? What is the maximum height? Round answers to the nearest tenth.

It will take 4 seconds to reach the maximum height of 288 feet.

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A toy rocket shot upward from the ground at a rate of 208 ft/sec has the quadratic equation of h = −16 t 2 + 208 t . When will the rocket reach its maximum height? What will be the maximum height? Round answers to the nearest tenth.

It will take 6.5 seconds to reach the maximum height of 676 feet.

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Access these online resources for additional instruction and practice graphing quadratic equations:

Key concepts

  • The graph of every quadratic equation is a parabola.
  • Parabola Orientation For the quadratic equation y = a x 2 + b x + c , if
    • a > 0 , the parabola opens upward.
    • a < 0 , the parabola opens downward.
  • Axis of Symmetry and Vertex of a Parabola For a parabola with equation y = a x 2 + b x + c :
    • The axis of symmetry of a parabola is the line x = − b 2 a .
    • The vertex is on the axis of symmetry, so its x -coordinate is − b 2 a .
    • To find the y -coordinate of the vertex we substitute x = − b 2 a into the quadratic equation.
  • Find the Intercepts of a Parabola To find the intercepts of a parabola with equation y = a x 2 + b x + c :
    y -intercept x -intercepts Let x = 0 and solve for y . Let y = 0 and solve for x .
  • To Graph a Quadratic Equation in Two Variables
    1. Write the quadratic equation with y on one side.
    2. Determine whether the parabola opens upward or downward.
    3. Find the axis of symmetry.
    4. Find the vertex.
    5. Find the y -intercept. Find the point symmetric to the y -intercept across the axis of symmetry.
    6. Find the x -intercepts.
    7. Graph the parabola.
  • Minimum or Maximum Values of a Quadratic Equation
    • The y - coordinate of the vertex of the graph of a quadratic equation is the
    • minimum value of the quadratic equation if the parabola opens upward.
    • maximum value of the quadratic equation if the parabola opens downward.

Practice makes perfect

Recognize the Graph of a Quadratic Equation in Two Variables

In the following exercises, graph:

y = x 2 + 3

This figure shows an upward-opening parabola graphed on the x y-coordinate plane. The x-axis of the plane runs from -10 to 10. The y-axis of the plane runs from -10 to 10. The parabola has a vertex at (0, 3) and goes through the point (1, 4).

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y = − x 2 + 1

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In the following exercises, determine if the parabola opens up or down.

y = −2 x 2 − 6 x − 7

down

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y = 6 x 2 + 2 x + 3

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y = 4 x 2 + x − 4

up

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y = −9 x 2 − 24 x − 16

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Find the Axis of Symmetry and Vertex of a Parabola

In the following exercises, find ⓐ the axis of symmetry and ⓑ the vertex.

y = x 2 + 8 x − 1

ⓐ x = −4 ⓑ ( −4 , −17 )

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y = x 2 + 10 x + 25

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y = − x 2 + 2 x + 5

ⓐ x = 1 ⓑ ( 1 , 6 )

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y = −2 x 2 − 8 x − 3

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Find the Intercepts of a Parabola

In the following exercises, find the x - and y -intercepts.

y = x 2 + 7 x + 6

y : ( 0 , 6 ) ; x : ( −1 , 0 ) , ( −6 , 0 )

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y = x 2 + 10 x − 11

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y = − x 2 + 8 x − 19

y : ( 0 , 19 ) ; x : none

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y = x 2 + 6 x + 13

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y = 4 x 2 − 20 x + 25

y : ( 0 , 25 ) ; x : ( 5 2 , 0 )

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y = − x 2 − 14 x − 49

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Graph Quadratic Equations in Two Variables

In the following exercises, graph by using intercepts, the vertex, and the axis of symmetry.

y = x 2 + 6 x + 5

y : ( 0 , 5 ) ; x : ( −1 , 0 ) , ( −5 , 0 ) ;
axis: x = −3 ; vertex : ( −3 , −4 )
This figure shows an upward-opening parabola graphed on the x y-coordinate plane. The x-axis of the plane runs from -10 to 10. The y-axis of the plane runs from -10 to 10. The parabola has points plotted at the vertex (-3, -4) and the intercepts (-5, 0), (-1, 0) and (0, 5). Also on the graph is a dashed vertical line representing the axis of symmetry. The line goes through the vertex at x equals -3.

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y = x 2 + 4 x − 12

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y = x 2 + 4 x + 3

y : ( 0 , 3 ) ; x : ( −1 , 0 ) , ( −3 , 0 ) ;
axis: x = −2 ; vertex: ( −2 , −1 )
This figure shows an upward-opening parabola graphed on the x y-coordinate plane. The x-axis of the plane runs from -10 to 10. The y-axis of the plane runs from -10 to 10. The parabola has points plotted at the vertex (-2, -1) and the intercepts (-1, 0), (-3, 0) and (0, 3). Also on the graph is a dashed vertical line representing the axis of symmetry. The line goes through the vertex at x equals -2.

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y = x 2 − 6 x + 8

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y = 9 x 2 + 12 x + 4

y : ( 0 , 4 ) x : ( − 2 3 , 0 ) ;
axis: x = − 2 3 ; vertex: ( − 2 3 , 0 )
This figure shows an upward-opening parabola graphed on the x y-coordinate plane. The x-axis of the plane runs from -5 to 5. The y-axis of the plane runs from -5 to 5. The parabola has points plotted at the vertex (-2 thirds, 0) and the intercept (0, 4). Also on the graph is a dashed vertical line representing the axis of symmetry. The line goes through the vertex at x equals -2 thirds.

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y = − x 2 + 8 x − 16

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y = − x 2 + 2 x − 7

y : ( 0 , −7 ) ; x : none ;
axis: x = 1 ; vertex : ( 1 , −6 )
This figure shows a downward-opening parabola graphed on the x y-coordinate plane. The x-axis of the plane runs from -10 to 10. The y-axis of the plane runs from -15 to 5. The parabola has points plotted at the vertex (1, -6) and the intercept (0, -7). Also on the graph is a dashed vertical line representing the axis of symmetry. The line goes through the vertex at x equals 1.

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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