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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