Pre-calc question?

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Hell0
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Pre-calc question?

Postby Hell0 » Sun Sep 01, 2013 6:37 pm

Hey!

A model rocket is launched. The height, in feet, of the rocket h(t) at t (t>=0) seconds after launch is determined by the equation h(t)= (-1/2)t^2 + 15t. The maximum height of the rocket is 112.5 feet.

(a) Find the number of seconds after launch it takes for the rocket to reach its maximum height. [Show all work]

(b) For how many seconds will the rocket be at a height of more than 100 ft? [Show all work]

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ok so its not like i tried it, i did, and let me show what i did.

first i plugged 112.5 for h(t) and got an answer of t=15 for part a.

for part b i plugged in 100 for h(t) and got 10 and 20. but i rejected 20 because thats higher than 15 which is the amount of time to reach the maximum height so that makes no sense. so i did 15 - 10 and got 5 for part b.

am i correct?
Last edited by Hell0 on Mon Sep 02, 2013 1:10 am, edited 1 time in total.

theshadow
Posts: 102
Joined: Sun Feb 22, 2009 11:12 pm

Re: Pre-calc question?

Postby theshadow » Sun Sep 01, 2013 7:24 pm

Hell0 wrote:The height...is determined by the equation h(t)= (-1/2t)^2 + 15t.

i think its really h(t) = -(1/2)t^2+15t. otherwise the power makes it 1/4t^2.

Hell0 wrote:first i plugged 112.5 for h(t) and got an answer of t=15 for part a.

i get that too

Hell0 wrote:for part b i plugged in 100 for h(t) and got 10 and 20. but i rejected 20 because thats higher than 15 which is the amount of time to reach the maximum height so that makes no sense.

why not? it goes up & then comes down right?

Hell0
Posts: 24
Joined: Sun Jun 23, 2013 5:04 am
Contact:

Re: Pre-calc question?

Postby Hell0 » Mon Sep 02, 2013 10:36 am

Hell0 wrote:for part b i plugged in 100 for h(t) and got 10 and 20. but i rejected 20 because thats higher than 15 which is the amount of time to reach the maximum height so that makes no sense.

theshadow wrote:why not? it goes up & then comes down right?

can you please explain further

theshadow
Posts: 102
Joined: Sun Feb 22, 2009 11:12 pm

Re: Pre-calc question?

Postby theshadow » Mon Sep 02, 2013 2:34 pm

Hell0 wrote:for part b i plugged in 100 for h(t) and got 10 and 20. but i rejected 20 because thats higher than 15 which is the amount of time to reach the maximum height so that makes no sense.
theshadow wrote:why not? it goes up & then comes down right?
Hell0 wrote:can you please explain further

if you throw something up it comes back down. the rocket was launched and then it comes back down. its gravity.


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