SOLUTION: H(t)= -5t^2 + at + b At time t=0, a ball was thrown upward from the top of a building. Before the ball hit the ground, its height h(t), in feet, at the time t seconds is given by

Algebra ->  Finance -> SOLUTION: H(t)= -5t^2 + at + b At time t=0, a ball was thrown upward from the top of a building. Before the ball hit the ground, its height h(t), in feet, at the time t seconds is given by       Log On


   



Question 1088880: H(t)= -5t^2 + at + b
At time t=0, a ball was thrown upward from the top of a building. Before the ball hit the ground, its height h(t), in feet, at the time t seconds is given by the function above. A and b are constants. If the ball reached its maximum height of 125 feet at time t=3, what could be the height of the building?

Answer by ikleyn(52771) About Me  (Show Source):
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H(t)= -5t^2 + at + b
At time t=0, a ball was thrown upward from the top of a building. Before the ball hit the ground, its height h(t), in feet,
at the time t seconds is given by the function above. A and b are constants.
If the ball reached its maximum height of 125 feet at time t=3, what could be the height of the building?
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Before starting my solution, I must make this notice.

    The form of your equation (the coefficient -5 at t^2) means that the units are (should be/must be) METERS, not feet.

    So, I will solve it assuming that the input is in METERS.

Solution

 
The fact that "the ball reached its maximum height of 125 feet at time t=3" means that the vertex form of the quadratic function H(t) is

H(t) = -5%28t-3%29%5E2+%2B+125.


It means that H(t) = -5t%5E2+%2B+30t+%2B+%28-5%2A9%29+%2B+125 = -5t%5E2+%2B+30t+-45+%2B+125 = -5t%5E2+%2B+30t+%2B+80. 


In turn, it means that the height of the building is 80 meters.

Answer. The height of the building is 80 meters.