I was reviewing the Coffin Algebra and Pre-calculus book for the 48GX and saw some trig identities and wondered about some of them. The first thing I see is some of the identities have = signs with three lines instead of two. Anyone know the significance of that?

Also, the half angle identities, when you take the sine of say, 20 degrees you get a number, a single number, and if you take the sine of half of that, sine 10 degrees, you get another single number. But when you look at the half angle identities, it goes +/- square root, etc, so you end up with two solutions. How can that be an identity when the original operation has only one solution?

Sin A/2 =( and for this one the = sign is a regular 2 bar one, why?)

+/- Sqrt (1-cos A/2)

So how can the dual solution value be the same as just dividing the angle by two and taking the sine which only has one solution?

Also as an aside, does anyone use the Mathpro card by Da Vinci Technologies group? How would you grade it compared to other math cards? Are there better ones, if so, which ones?

I have a Mathpro card installed in my 48GX and it sure seems powerful to me but don't know the extent of the competition. Thanks all. Don.