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					Lesson 7.4
   Although you can always use a graph of a
    quadratic function to approximate the x-
    intercepts, you are often not able to find
    exact solutions. This lesson will develop a
    procedure to find the exact roots of a
    quadratic equation. Consider again this
    situation from Example C in the previous
    lesson.
   Nora hits a softball straight up at a speed of 120
    ft/s. Her bat contacts the ball at a height of 3 ft
    above the ground. Recall that the equation relating
    height in feet, y, and time in seconds, x, is
    y=-16x2 +120x+3. How long will it be until the
    ball hits the ground?
   The height will be zero when the ball hits the
    ground, so you want to find the solutions to the
    equation -16x2 +120x+3=0. You can approximate
    the x-intercepts by graphing, but you may not be
    able to find the exact x-intercept.
   You will not be able to factor this equation using a rectangle
    diagram, so you can’t use the zero-product property.
   Instead, to solve this equation symbolically, first write the
    equation in the form a(x-h)2+k=0.

                                                 The zeros of the
                                                 function are
                                                 x=7.525 and
                                                 x=- 0.025.
                                                 The negative time,
                                                 -0.025 s, does not
                                                 make sense in this
                                                 situation, so the ball
                                                 hits the ground after
                                                 approximately
                                                 7.525 s.
   If you follow the
    same steps with a
    general quadratic
    equation, then you
    can develop the
    quadratic formula.
   This formula provides
    solutions to ax2 +bx + c
    =0 in terms of a, b, and
    c.
   To use the quadratic formula on the equation
    in Example A, 16x2+120x +3= 0, first
    identify the coefficients as a=16, b=120, and
    c=3. The solutions are
   Salvador hits a baseball at a height of 3 ft and
    with an initial upward velocity of 88 feet per
    second.
    ◦ Let x represent time in seconds after the ball is hit,
      and let y represent the height of the ball in feet.
      Write an equation that gives the height as a
      function of time.
    ◦ Y=-16x2+88x+3
   Write an equation to find the times when the
    ball is 24 ft above the ground.
    ◦ 24=-16x2+88x +3
   Rewrite your equation from the previous
    step in the form, ax2 +bx + c =0 then use
    the quadratic formula to solve. What is the
    real-world meaning of each of your
    solutions? Why are there two solutions?
    ◦ -16x2+88x -21=0
    ◦ X=0.25 or x=5.25.
    ◦ The ball rises to 24 ft on the way up at 0.25 s, and it falls
      to 24 ft on the way down at 5.25 s.
 Find the y-coordinate of the vertex of this
 parabola.
 How many different x-values correspond to
 this y-value? Explain.
    ◦ 124
    ◦ The ball reaches the maximum height only once.
      The ball reaches other heights once on the way up
      and once on the way down, but the top of the ball’s
      height, the maximum point, can be reached only
      once.
   Write an equation to find the time when the ball
    reaches its maximum height.
    ◦ Use the quadratic formula to solve the equation.
      At what point in the solution process does it
      become obvious that there is only one solution to
      this equation?
       -16x2 +88x +3=124; x=2.75; when b2  4ac  0
   Write an equation to find the time when the ball
    reaches a height of 200 ft. What happens when
    you try to solve this impossible situation with the
    quadratic formula?
    -16x2 +88x +3= 200. You get the square root of a
    negative number, so there are no real solutions.
   Solve 3x2=5x+8.
   To use the quadratic formula, first write the equation in the
    form ax2+bx+c=0 and identify the coefficients: 3x2-5x-8=0
   a=3, b=5, c=8
   Substitute a, b, and c into the quadratic formula.
   The solutions are
    x=8/3 or x=-1.
Remember, you can find exact solutions to
   some quadratic equations by factoring.
  However, most quadratic equations don’t
 factor easily. The quadratic formula can be
    used to solve any quadratic equation.

				
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posted:10/4/2012
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