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A toy rocket is launched from the ground, and its height (in meters) above the ground at any time [tex]$t$[/tex] (in seconds) is given by the equation:

[tex]$h(t) = -5t^2 + 20t + 15$[/tex]

Where [tex]$h(t)$[/tex] is the height of the rocket. How long will it take for the rocket to reach the ground?

Answer :

To find out how long it will take for the toy rocket to reach the ground, we need to determine when its height is 0 meters. The height of the rocket is given by the equation:

[tex]\[ h(t) = -5t^2 + 20t + 15 \][/tex]

This is a quadratic equation in the form of [tex]\( ax^2 + bx + c = 0 \)[/tex], where [tex]\( a = -5 \)[/tex], [tex]\( b = 20 \)[/tex], and [tex]\( c = 15 \)[/tex].

To find the time [tex]\( t \)[/tex] when the rocket reaches the ground, we set [tex]\( h(t) = 0 \)[/tex]:

[tex]\[ -5t^2 + 20t + 15 = 0 \][/tex]

We can solve this quadratic equation using the quadratic formula:

[tex]\[ t = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \][/tex]

Plug in the values of [tex]\( a \)[/tex], [tex]\( b \)[/tex], and [tex]\( c \)[/tex]:

[tex]\[ t = \frac{-20 \pm \sqrt{20^2 - 4 \cdot (-5) \cdot 15}}{2 \cdot (-5)} \][/tex]

[tex]\[ t = \frac{-20 \pm \sqrt{400 + 300}}{-10} \][/tex]

[tex]\[ t = \frac{-20 \pm \sqrt{700}}{-10} \][/tex]

This simplifies to:

[tex]\[ t = \frac{-20 \pm \sqrt{700}}{-10} \][/tex]

The two values of [tex]\( t \)[/tex] are:

[tex]\[ t = \frac{-20 + \sqrt{700}}{-10} \][/tex] and [tex]\[ t = \frac{-20 - \sqrt{700}}{-10} \][/tex]

These calculations yield two solutions:

1. [tex]\( t = 2 - \sqrt{7} \)[/tex]
2. [tex]\( t = 2 + \sqrt{7} \)[/tex]

The time when the rocket hits the ground is a positive value, so we choose the positive solution:

[tex]\[ t = 2 + \sqrt{7} \][/tex]

Thus, the rocket will reach the ground approximately [tex]\( 2 + \sqrt{7} \)[/tex] seconds after it is launched.

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