ActionScript 3.0 :: Maths: Exponential Value Via Loop?

To create a sort of thick-pixel gradient shadow, I've got a simple loop which creates a new 200x5 pixel Shape (rectangle) with each iteration, assigning an alpha value and y value that changes relative to the loop's iteration.

Code:
var upperShadow:Shape = new Shape();
for (var k:int=0; k<10; k++) {
shadowAlpha = (1-(k/10))*.5;
trace(shadowAlpha); // traces .5, .45, .4, .35, .3, .25, .2, .15, .1, .05

[code]....

This creates a simple linear gradient since the alpha value changes linearly, but how would I go about creating some sort of exponential/logarithmic/cubic/circular value for the alpha, so that it falls off differently?

Are there a set of common equations that people use for this sort of thing, similar to the simple (1-(k/10))*.5 I use for the linear change from .5 to 0 in ten .5 increments?