## Sunday, June 21, 2009

### Trig identites and multiplication

We're used to the idea of using logarithms to convert multiplication problems into addition problems, because of the relation:

log(xy) = log(x) + log(y)

So if you have a table of logarithms, to multiply x and y you can look up the logarithms of each, add them, then find the number in your tables whose logarithm is that sum. Three table-lookups and an addition. [Well, in practice you do the last lookup in a table of antilogarithms, but that's only a minor benefit, it's not actually necessary to the calculation.]

However, there are other ways to make multiplication "simpler" than trying to directly multiply, such as this identity:

cos x cos y = [cos (x + y) + cos (x - y)]/2

This sort of relationship was actually used to do multiplication; it's a bit more complicated than logarithms, but it works well enough if you haven't invented logs yet, but do have cosine tables. It works like this.

You want to multiply two numbers, X and Y (and let's assume that they have already been scaled to lie in a range where this will work).

You find the numbers that they are the cosines of in your tables, X = cos x, Y = cos y. Add and difference those two numbers, giving (x+y) and (x-y). Look those numbers up in your cosine tables. Average them. That's the product. Four table lookups, an addition and a halving (and some rescaling back to the original problem, presumably, which probably just involves moving a decimal point), all much easier than general multiplication.

So far so good.

But here's what I am wondering. Humans came up with a nifty device to automate that multiplication via addition of logs, the slide rule. Is it possible to build a device that does mutliplication using a trig identity like the one above, perhaps some sort of "swivel rod"?

If we do it in two parts, I think maybe we can do it with a slide rule; one side does the additions and subtraction (and halving; halving is inverse doubling, which is just addition, so it can also be done there). The other does the conversions to and from normal scale to cosine scale - but you'd be forever transferring numbers across the different scales.

So I wonder if it can all be combined in a single, simpler system, whether it's a kind of slide rule or something more complex involving actual rotations to do the cosines (accuracy may be an issue though).

I don't think a device was ever made that did this, but I think that it might be possible.

If it were, I'd love to have one.