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An object is thrown upward at a speed of 194 feet per second by a machine from a height of 15 feet off the ground. The height [tex]\(h\)[/tex] of the object after [tex]\(t\)[/tex] seconds can be found using the equation:
[tex]\[ h = -16t^2 + 194t + 15 \][/tex]

1. When will the height be 413 feet?
[tex]\[ \square \text{ seconds} \][/tex]
Hint: Set [tex]\(h\)[/tex] to 413.

2. When will the object reach the ground?
[tex]\[ \square \text{ seconds} \][/tex]


Sagot :

To solve the given problems, we'll use the equation for the height of the object, which is [tex]\( h = -16t^2 + 194t + 15 \)[/tex]. We'll solve for specific values of [tex]\( t \)[/tex] in two scenarios: when the height [tex]\( h \)[/tex] is 413 feet and when the object reaches the ground (i.e., the height [tex]\( h = 0 \)[/tex]).

### 1. Finding when the height will be 413 feet:

We start by setting the height [tex]\( h \)[/tex] to 413 feet and solve for [tex]\( t \)[/tex] in the equation:

[tex]\[ 413 = -16t^2 + 194t + 15 \][/tex]

Rearranging the equation to bring all terms to one side, we get:

[tex]\[ -16t^2 + 194t + 15 - 413 = 0 \][/tex]
[tex]\[ -16t^2 + 194t - 398 = 0 \][/tex]

This is a quadratic equation of the form [tex]\( at^2 + bt + c = 0 \)[/tex], where [tex]\( a = -16 \)[/tex], [tex]\( b = 194 \)[/tex], and [tex]\( c = -398 \)[/tex]. Solving this quadratic equation using the quadratic formula [tex]\( t = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \)[/tex]:

[tex]\[ t = \frac{-194 \pm \sqrt{194^2 - 4(-16)(-398)}}{2(-16)} \][/tex]
[tex]\[ t = \frac{-194 \pm \sqrt{37636 - 25472}}{-32} \][/tex]
[tex]\[ t = \frac{-194 \pm \sqrt{12164}}{-32} \][/tex]
[tex]\[ t = \frac{-194 \pm 110.3126}{-32} \][/tex]

This gives us two solutions:

[tex]\[ t_1 = \frac{-194 + 110.3126}{-32} = \frac{-83.6874}{-32} = 2.615 \text{ seconds} \][/tex]
[tex]\[ t_2 = \frac{-194 - 110.3126}{-32} = \frac{-304.3126}{-32} = 9.510 \text{ seconds} \][/tex]

So, the height will be 413 feet at approximately [tex]\( 2.615 \)[/tex] seconds and [tex]\( 9.510 \)[/tex] seconds.

### 2. Finding when the object will reach the ground:

To determine when the object reaches the ground, we set the height [tex]\( h \)[/tex] to 0 and solve for [tex]\( t \)[/tex]:

[tex]\[ 0 = -16t^2 + 194t + 15 \][/tex]

This is again a quadratic equation. Solving for [tex]\( t \)[/tex] using the quadratic formula:

[tex]\[ t = \frac{-194 \pm \sqrt{194^2 - 4(-16)(15)}}{2(-16)} \][/tex]
[tex]\[ t = \frac{-194 \pm \sqrt{37636 + 960}}{-32} \][/tex]
[tex]\[ t = \frac{-194 \pm \sqrt{38596}}{-32} \][/tex]
[tex]\[ t = \frac{-194 \pm 196.4533}{-32} \][/tex]

This gives us two solutions:

[tex]\[ t_1 = \frac{-194 + 196.4533}{-32} = \frac{2.4533}{-32} = -0.077 \text{ seconds} \][/tex]
[tex]\[ t_2 = \frac{-194 - 196.4533}{-32} = \frac{-390.4533}{-32} = 12.202 \text{ seconds} \][/tex]

Therefore, because time cannot be negative, the object will reach the ground at approximately [tex]\( 12.202 \)[/tex] seconds.

### Summary:

- The object will be 413 feet high at approximately [tex]\( 2.615 \)[/tex] seconds and [tex]\( 9.510 \)[/tex] seconds.
- The object will reach the ground at approximately [tex]\( 12.202 \)[/tex] seconds.
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