File this hint from NaOH-Lye under "it seems so obvious, but did you know?":
As you probably do know, Spotlight can serve as a calculator. If, for example, you enter 100+4 in the Spotlight query box and hit Return, you'll get the usual list of Spotlight results. At the top: A result from Calculator, 100+4 = 104.
What you might not have known: With this result selected, press Command-C to copy the result of the calculation to the Clipboard. From there, you can paste it wherever you like.
Hints commenters pointed out that this same thing works with other Spotlight results. (Search for and find a file, for example, and you can copy it from the Spotlight results list then paste the file itself into a Finder window or the file path into a text file.) They also remind us (per this hint) that, since 10.5, the Spotlight calculator has supported a variety of math functions and constants, including sqrt, log, pi, and more.
This story, "Copy Math Results From Spotlight" was originally published by Macworld.