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Below is an "oracle" function. An oracle function is a function presented interactively. When you type in an $t$ value, and press the --f--$>$ button and the value $f(t)$ appears in the right hand window. There are three lines, so you can easily calculate three different values of the function at one time.

The function $f(t)$ represents the height in feet of a ball thrown into the air, $t$ seconds after it has been thrown. Calculate the velocity 0.8 seconds after the ball has been thrown.

After 0.8 seconds, the velocity is

tf(t)

Remember this technique for finding velocities. Later we will use the same method to find the derivative of functions such as $f(t)$.