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