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Re: why is this happening?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg99259] Re: [mg99198] why is this happening?
*From*: Bob Hanlon <hanlonr at cox.net>
*Date*: Fri, 1 May 2009 05:22:25 -0400 (EDT)
*Reply-to*: hanlonr at cox.net
If you want Mathematica to assume that k1 is positive you must explicitly do so
soln = Solve[k2/k1 + ld^2 + ld == 0, ld] // FullSimplify
{{ld->-(Sqrt[k1-4 k2]/(2 Sqrt[k1]))-1/2},{ld->1/2 (Sqrt[k1-4 k2]/Sqrt[k1]-1)}}
FullSimplify[soln, k1 > 0]
{{ld->-1/2 Sqrt[1-(4 k2)/k1]-1/2},{ld->1/2 (Sqrt[1-(4 k2)/k1]-1)}}
Bob Hanlon
---- sean_incali at yahoo.com wrote:
=============
Consider the following.
Solve[k1 ld + k1 k2 + ld^2 == 0, ld]
vs
Solve[k2/k1 + ld^2 + ld == 0, ld]
I expected to get the output I got from the first code.
>From the second code, I was expecting something along the lines of
ld ->-1 + (SquareRoot[1 - 4 (k2/k21)])/2
ld -> -1 - (SquareRoot[1 - 4 (k2/k21)])/2
since that is the answer.
Why is Mathematica turning the second scaled equation into the
unscaled one in the first before solving it?
Thanks for any insight.
Sean
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