High School

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A toy rocket is launched from a platform that is 48 feet high. The rocket's height above the ground is modeled by [tex]h = -16t^2 + 32t + 48[/tex].

a. Find the maximum height of the rocket.

b. Find the time it will take for the rocket to reach the ground.

Answer :

We are given the height function

[tex]$$
h(t) = -16t^2 + 32t + 48
$$[/tex]

for the rocket, where [tex]$t$[/tex] is time in seconds and [tex]$h(t)$[/tex] is the height in feet.

Step 1. Determine the time when the rocket reaches its maximum height.
For a quadratic function of the form

[tex]$$
ax^2 + bx + c,
$$[/tex]

the vertex (which gives the maximum for a downward opening parabola, [tex]$a < 0$[/tex]) occurs at

[tex]$$
t = -\frac{b}{2a}.
$$[/tex]

Here, [tex]$a = -16$[/tex] and [tex]$b = 32$[/tex]. Thus,

[tex]$$
t = -\frac{32}{2(-16)} = -\frac{32}{-32} = 1.
$$[/tex]

This means the maximum height occurs at [tex]$t = 1$[/tex] second.

Step 2. Find the maximum height.
Substitute [tex]$t = 1$[/tex] into the height function:

[tex]$$
h(1) = -16(1)^2 + 32(1) + 48 = -16 + 32 + 48 = 64.
$$[/tex]

So, the maximum height is [tex]$\boxed{64}$[/tex] feet.

Step 3. Determine the time when the rocket reaches the ground.
The rocket reaches the ground when [tex]$h(t) = 0$[/tex]. Set the height equation equal to zero:

[tex]$$
-16t^2 + 32t + 48 = 0.
$$[/tex]

It can be easier to work with positive leading coefficients. Multiply the entire equation by [tex]$-1$[/tex]:

[tex]$$
16t^2 - 32t - 48 = 0.
$$[/tex]

Step 4. Solve the quadratic equation using the quadratic formula:

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

In the equation [tex]$16t^2 - 32t - 48 = 0$[/tex], we identify [tex]$a = 16$[/tex], [tex]$b = -32$[/tex], and [tex]$c = -48$[/tex]. Compute the discriminant:

[tex]$$
\Delta = (-32)^2 - 4(16)(-48) = 1024 + 3072 = 4096.
$$[/tex]

Taking the square root gives:

[tex]$$
\sqrt{\Delta} = 64.
$$[/tex]

Now, solve for [tex]$t$[/tex]:

[tex]$$
t = \frac{-(-32) \pm 64}{2 \times 16} = \frac{32 \pm 64}{32}.
$$[/tex]

This yields two solutions:

1. When choosing the positive square root:

[tex]$$
t = \frac{32 + 64}{32} = \frac{96}{32} = 3.
$$[/tex]

2. When choosing the negative square root:

[tex]$$
t = \frac{32 - 64}{32} = \frac{-32}{32} = -1.
$$[/tex]

Since time cannot be negative, we discard [tex]$t = -1$[/tex]. Thus, the rocket reaches the ground at [tex]$\boxed{3}$[/tex] seconds.

Final Answer:
a. The maximum height of the rocket is [tex]$64$[/tex] feet.
b. The rocket reaches the ground after [tex]$3$[/tex] seconds.

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