[tuatoru]

The Pythagorean theorem is really useful for any construction. Builders use it regularly. More experienced DIYers also use it. Not a good example of rarely used schoolbook learning.

How about obscure stuff about hyperbolic trig functions (sinh, tanh and so on)?

[voidhorse]

I actually just had to use some sin/cos the other day while programming some audio stuff so there ya go. Not so useless after all.

[alanbernstein]

I remember telling a friend once in high school, that I was kind of happy to find a good real-world use for cubic polynomials. She asked what it was, so I started "well I was programming..." and she interrupted, "of course math is useful for programming, that doesn't count".