Answer:
About 1.85 seconds and 13.15 seconds.
Step-by-step explanation:
The height (in feet) of the rocket <em>t</em> seconds after launch is given by the equation:
And we want to determine how many seconds after launch will be rocket be 390 feet above the ground.
Thus, let <em>h</em> = 390 and solve for <em>t: </em>
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Isolate:
Simplify:
We can use the quadratic formula:
In this case, <em>a</em> = 8, <em>b</em> = -120, and <em>c</em> = 195. Hence:
Evaluate:
Simplify:
Thus, our two solutions are:
Hence, the rocket will be 390 feet above the ground after about 1.85 seconds and again after about 13.15 seconds.