Answer:
The river is 6 feet at two times, 2.15 hours after the rain and 4.65 hours after the rain.
Step-by-step explanation:
We are given the following in the question:
[tex]d=-0.25t^2+1.7t+3.5[/tex]
[tex]0\leq t\leq 7[/tex]
where, d is the depth of river in feet and t is time in hours after a heavy rain.
We have to find the number of hours for which the depth of river is 6 feet.
Putting d = 6 in the equation, we get,
[tex]6=-0.25t^2+1.7t+3.5\\\Rightarrow +0.25t^2-1.7t+2.5 = 0\\\text{Using quadratic formula}\\\\\Rightarrow t = \dfrac{1.7\pm \sqrt{(-1.7)^2-4(0.25)(2.5)}}{2(0.25)}\\\\t\approx 4.65, 2.15[/tex]
Thus, the river is 6 feet at two times, 2.115 hours after the rain and 4.65 hours after the rain.
The attached image shows the graph.
