SOLUTION: Building on question 95079, I have the part that goes like this:The only part of the height equation that we have not discussed is the constant. You have probably noticed that the

Question 102209: Building on question 95079, I have the part that goes like this:The only part of the height equation that we have not discussed is the constant. You have
probably noticed that the constant is always equal to the initial height of the ball (80 m in
our previous exercises). Now, let�s have you develop a height equation.
A ball is thrown upward from the roof of a building 100 m tall with an initial velocity
of 20 m/s. Use this information for exercises 35 to 38.
35. Science and medicine. Write the equation for the height of the ball.
36. Science and medicine. When will the ball reach a height of 80 m?
I am trying to solve #36. So far I have:
h=-5t^2+20t+100
0=t^2-4-20
0= 4+or-sqrt-5^2-4*1*-20/2*1
=4+or-sqrt16+80/2
4=+or-sqrt96/2
4+9.8/2=13.8/2=6.9
4-9.8/2=5.8/2=2.9
am I anywhere near where I need to be?
Thank you so much.

Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
Start with the given equation (assuming you did everything right for problem 35)

When the height is 80, this means h=80

So plug in h=80

Subtract 80 from both sides

Let's use the quadratic formula to solve for t:

Starting with the general quadratic

the general solution using the quadratic equation is:

So lets solve ( notice , , and )

Plug in a=-5, b=20, and c=20

Square 20 to get 400

Multiply to get

Combine like terms in the radicand (everything under the square root)

Simplify the square root (note: If you need help with simplifying the square root, check out this solver)

Multiply 2 and -5 to get -10

So now the expression breaks down into two parts

or

Now break up the fraction

or

Simplify

or

So these expressions approximate to

or

So our possible solutions are:
or

However, since a negative time doesn't make sense, our only solution is

So at about 4.8 seconds the ball will be 80 m high

RELATED QUESTIONS
The only part of the height equation we have not discussed is the constant. You have... (answered by renevencer22)

Original equation= h=-5t^2+25t+80 The only part of the height equation that we have... (answered by stanbon)

I would like your help with the following question in year 7 maths:- A girl is 11... (answered by SHUgrad05)

The only part of the height equation we have not discussed is the constant. You have... (answered by stanbon)

please help me not just solve it but understand the following question because i don't... (answered by josgarithmetic,MathTherapy,ikleyn)

Hi, could you please help me on a problem that I have. I do not need to solve this... (answered by rapaljer)

Hi, my class just started studying pyramids and I need help on a problem. I'm really... (answered by Earlsdon)

I have a word problem that goes like this: If I have a dart game and only 4 points or (answered by AnlytcPhil)

You are standing 75 meters from the base of the Jin Mao in Shangai, China. You estimate... (answered by rfer)