# Homework Help: Equation (Chandrasekhar, Newton's Principia)

1. Feb 22, 2016

### Poetria

1. The problem statement, all variables and given/known data

I am reading a book by Chandrasekhar, "Newton's Principia for a Common Reader." I don't understand some notation.

2. Relevant equations

(2) A(1)S(1) x A(1)D(1)=(A(1)B(1))^2

What does x means here? A cross product? Could you give me a hint where to find a good introduction to these geometrical operations? Obviously, there is a gap in my background.

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2. Feb 22, 2016

### Ray Vickson

It looks like a simple "times" = "multiplication" sign.

3. Feb 22, 2016

### Poetria

Oh, ok. I am overegging the pudding then. :) Many thanks. :) Sometimes he doesn't use it you know.

4. Feb 22, 2016

### Ray Vickson

Just for future reference: the × sign could not be a cross-product, because that would give you an equation with a vector on one side and a scalar on the other. Besides, Newton wrote Principia hundreds of years before the invention of vectors and cross-products, etc (although, of course, maybe Chandraskhar is using modern notation and concepts in writing about Newton's work).

5. Feb 23, 2016

### Poetria

Indeed he does use modern notation. And in the precedent chapter he employs the same sign with vectors. In addition, he refers to the concept of versed sine, which is little used but fortunately I have found a good definition.
I have also found "A History of Vector Analysis." :)
Thank you very much. It is tough stuff. :)