Functions are machines. Plug the independent variable into the machine and it spits out the dependent variable.

If *y* = *f*(*x*) = *x* + 1, then as *x* gets larger (moves right), *y* gets larger also (moves up). As *x* gets smaller (moves left), *y* gets smaller also (moves down).

Here's something to play with: see what happens to *y* as we make *x* larger or smaller.

If *y* = *f*(*x*) = 1 - *x*, then as *x* gets larger (moves right), *y* gets smaller (moves down). As *x* gets smaller (moves right), *y* gets larger (moves up).

Say *y* = *f*(*x*) = *x*^{2}. We start *x* at -5. As *x* moves right, *y* gets smaller until *x* reaches 0. If, starting at 0, we keep moving *x* to the right, *y* starts getting bigger again.

With this function, in order to say whether *y* is increasing or decreasing as we play with *x*, we need to know two things: whether *x* is to the left or the right of zero, and whether *x* is being moved to the right or left.

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