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Vint is testing ceiling fans in a factory. For one of the tests, he switches the fan on, and after it attains a maximum speed of 500 rotations per minute (rpm), he switches the fan back off, recording the amount of time it takes for the fan to completely stop spinning. The given equation models Vint's test, where [tex]x[/tex] represents time in seconds and [tex]y[/tex] represents the speed in rotations per minute.

[tex]y = -5x^2 + 100x[/tex]

Determine when the test was complete, that is, when the fan stopped spinning completely. How long was the test?

Answer :

We begin with the model for the fan test:

[tex]$$
y = -5x^2 + 100x,
$$[/tex]

where [tex]$x$[/tex] is the time in seconds and [tex]$y$[/tex] is the speed in rotations per minute.

Step 1. Set the speed equal to zero.
When the fan stops, [tex]$y = 0$[/tex], so:

[tex]$$
-5x^2 + 100x = 0.
$$[/tex]

Step 2. Factor the equation.
We factor out the common term [tex]$-5x$[/tex]:

[tex]$$
-5x(x - 20) = 0.
$$[/tex]

The equation is now already separated into factors.

Step 3. Solve for [tex]$x$[/tex].
For the product to be zero, either factor must be zero:

1. [tex]$-5x = 0$[/tex] leads to [tex]$x = 0$[/tex],
2. [tex]$x - 20 = 0$[/tex] leads to [tex]$x = 20$[/tex].

Since [tex]$x = 0$[/tex] corresponds to the initial moment when the fan is turned on, we ignore it. The meaningful solution is:

[tex]$$
x = 20.
$$[/tex]

Step 4. Interpret the result.
The fan comes to a complete stop after 20 seconds.

For completeness, the quadratic equation has a discriminant given by:

[tex]$$
D = b^2 - 4ac,
$$[/tex]

where [tex]$a = -5$[/tex], [tex]$b = 100$[/tex], and [tex]$c = 0$[/tex]. After calculation:

[tex]$$
D = 100^2 - 4(-5)(0) = 10000.
$$[/tex]

The roots obtained using the quadratic formula confirm the times [tex]$x=0$[/tex] and [tex]$x=20$[/tex] seconds.

Conclusion:
The fan takes [tex]$\boxed{20}$[/tex] seconds to completely stop.

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Rewritten by : Barada