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#1 2007-02-04 23:38:27

Toast
Real Member
Registered: 2006-10-08
Posts: 1,321

Some formulae

Simple rearranging here, but I think I mighta made a mistake (or two)

Solve for s:









...

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#2 2007-02-05 01:34:15

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

Re: Some formulae

You went wrong in the step when you multiplied everything by s. The last term should just be cr, not crs.

If you follow through from there then you get:

rsv = cs - cr
rsv - cs = -cr
s(rv-c) = -cr
s = cr/(c-rv)


Why did the vector cross the road?
It wanted to be normal.

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#3 2007-02-05 21:41:35

Dross
Member
Registered: 2006-08-24
Posts: 325

Re: Some formulae

Toast wrote:

Solve for s:









...

I think it's worth noting what you've done in the last three steps - as soon as you get to the step:

you've divided through by

and decided that
- but remember that you always have to be sure you're not dividing by zero. However, because
, then
or
(or both) - but you can't conclude either one of them is zero until you know for sure that they're both zero, or that one of them is not zero (for example, if you know s is always positive).

See what I'm saying?


Bad speling makes me [sic]

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#4 2007-02-06 00:36:01

Toast
Real Member
Registered: 2006-10-08
Posts: 1,321

Re: Some formulae

Ah, yep, thanks both. If I divide through it'll be no use for quadratics ;o

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