The hardest math problem

[joeorez]

We don't use much trig in actuarial work?

I guess you don't want to see my Boiler & Machinery trend factors.

[phdmom]

IF a/b = c/d, THEN a*d = b*c.

The moral: if a/b = c/d is the thing you are trying to prove, don't cross-multiply. Why not? Because you can't go backwards. In other words, it is possible for a*d = b*c without a/b = c/d.

If you're trying to prove an identity, start with one side of the equation and simplify it till you get the other side (someone said that, I know).

I loved teaching trig.