Arh, I understand your confusion. This result does work for this quadratic, but is not true in general. If you multiply out the expression you'll see that it is true.
The reason is that r1 and r2 are reciprocals; ie. r1 = 1/r2 and r2 = 1/r1
If you make that substitution you'll see that the expression becomes (1-r1)(1-r2) which is what you have.
Hopefully that's enough for you to sort this out. I'm in a rush right now, but I'll post more details later if you need.
Bob
]]>The book result comes straight from the quadratic formula http://www.mathsisfun.com/algebra/quadr … ation.html
But your result is the same. You just have a negative on both sides of the book version.
Bob
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