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Below is an "oracle" function. An oracle function is a function presented interactively. When you type in a $t$ value, and press the --f--$>$ button the value $f(t)$ appears in the right hand window. There are three lines, so you can 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 instantaneous velocity 0.47 seconds after the ball has been thrown.

Instantaneous velocity at $0.47$ =

tf(t)