SOLUTION: I stand on a building 640 feet high and throw a rock into the air with a velocity of 96 ft/sec. When does the rock have a height of 100 ft?

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Question 763173: I stand on a building 640 feet high and throw a rock into the air with a velocity of 96 ft/sec. When does the rock have a height of 100 ft?
Answer by Alan3354(69443) (Show Source):
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I stand on a building 640 feet high and throw a rock into the air with a velocity of 96 ft/sec. When does the rock have a height of 100 ft?
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Use the common function for height vs time on Earth,
h in feet, t in seconds

Solved by pluggable solver: SOLVE quadratic equation (work shown, graph etc)

Quadratic equation (in our case ) has the following solutons:

For these solutions to exist, the discriminant should not be a negative number.

First, we need to compute the discriminant : .

Discriminant d=2736 is greater than zero. That means that there are two solutions: .

Quadratic expression can be factored:

Again, the answer is: 9.53834841531101, -3.53834841531101. Here's your graph:

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Ignore the negative solution
t = 9.538 seconds