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2 years ago

Solve −2x2 +3x − 9 = 0

1 answer:

8 0

-2x^2 + 3x - 9 = 0
The quadratic is not factorable so quadratic formula must be used.
x = (-b + - √(b^2 - 4ac))/2a
a = -2, b = 3, c = -9
x = (-3 + - √(9 - 4*-2*-9))/-4
x = (-3 + - √(-63))/-4
x = -3 + - 3i√7
   --------------
   -4
x = 3 + 3i√7 x = 3 - 3i√7
    ------------ ------------
   4 4

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Jennifer serves the volleyball to Marsha with an upward velocity of 10.5 ft/s. The ball is 5 feet above the ground when she stri

Katarina [22]

Answer:

0.98 seconds

Step-by-step explanation:

We assume the height of the volleyball is described by the equation for ballistic motion. We want to find the time it takes for the height to become zero.

<h3>motion equation</h3>

The general form of the equation of height for ballistic motion is ...

$h(t)=-16t^2+v_0t+h_0\qquad\text{$v_0$ and $h_0$ are the initial velocity and height}$

The coefficient 16 in the equation is an approximation of 1/2g, where g is the acceleration due to gravity in ft/s². This means the units of time and distance are expected to be seconds and feet.

For the problem at hand, the initial velocity and height are 10.5 ft/s and 5 ft. Then the height equation is ...

h(t) = -16t² +10.5t +5

<h3>reaction time</h3>

Marsha has until the ball hits the ground to react to the serve. To find out how long that is, we need to solve the height equation for t when h=0. This is most easily done using the quadratic formula with ...

a = -16
b = 10.5
c = 5

The solution is ...

$t=\dfrac{-b\pm\sqrt{b^2-4ac}}{2a}= \dfrac{-10.5\pm\sqrt{10.5^2-4(-16)(5)}}{2(-16)}\\\\=\dfrac{10.5\pm\sqrt{430.25}}{32}=\dfrac{21\pm\sqrt{1721}}{64}$

The positive solution is ...

t ≈ 0.976327 ≈ 0.98

Marsha has about 0.98 seconds to react before the volleyball hits the ground.

_____

<em>Additional comment</em>

After about 0.33 seconds, Marsha knows she doesn't need to react at all. The serve will not clear the net. Its maximum height is about 6' 8 5/8". A women's volleyball net is 7' 4 1/8" high. Jennifer's serve velocity must be at least 12.3 ft/s for the ball to go over the net. With that upward velocity, Marsha has about 1.06 seconds to react.

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