Math Is Fun Forum

demha

Member

Registered: 2012-11-25

Posts: 195

Quick Question

I was given this word problem:

Suppose a football is kicked from the ground and its height in feet above the ground is given by h = -3.9t^2 + 15.6t. The height above the ground is "h". The time in seconds, after the ball is kicked, is "t". How long is the ball in the air?
A 10 seconds
B 5 seconds
C 18 seconds
D 4 seconds
E 15.6 seconds
F 3.9 seconds

I took
-3.9t^2 + 15.6t
and then factored -1t to be left with:
-1t(3.9t + 15.6)

From here I'm not all too sure where to go.

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"The thing about quotes on the Internet is you cannot confirm their validity"
~Abraham Lincoln

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: Quick Question

Hm, have you tried completing tbe square? You will see for which value of t, h reaches its maximum. The total time is twice as long.

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“Here lies the reader who will never open this book. He is forever dead.
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Bob

Administrator

Registered: 2010-06-20

Posts: 10,134

Re: Quick Question

hi demha

-1t(3.9t - 15.6)

I've corrected the sign mistake here.

This is enough to solve this problem.  h = 0 at two places; the start and the finish.

so 3.9t - 15.6 = 0 will give you the time of flight.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

demha

Member

Registered: 2012-11-25

Posts: 195

Re: Quick Question

So with that I'll just solve for t and get my answer,correct?

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"The thing about quotes on the Internet is you cannot confirm their validity"
~Abraham Lincoln

Bob

Administrator

Registered: 2010-06-20

Posts: 10,134

Re: Quick Question

Yes,

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

demha

Member

Registered: 2012-11-25

Posts: 195

Re: Quick Question

3.9t - 15.6 = 0

SO we are solving for 't' here. Reordering:

-15.6 + 3.9t = 0

Add 15.6 to both sides:

15.6 + (-15.6) + 3.9t = 0 + 15.6

3.9t = 15.6

Divide each side by 3.9 for a final answer of:

t = 4

Answer: 'D'

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"The thing about quotes on the Internet is you cannot confirm their validity"
~Abraham Lincoln