Re: solving a system of two equations

*To*: mathgroup at smc.vnet.net*Subject*: [mg102008] Re: [mg101984] solving a system of two equations*From*: Tomas Garza <tgarza10 at msn.com>*Date*: Sun, 26 Jul 2009 03:55:57 -0400 (EDT)*References*: <200907250817.EAA29655@smc.vnet.net>

Try this (I guess you forgot to write the second variable, b, in the NSolve statement): In[20]:= sol = NSolve[{a/(a+b)==0.5, a*b/((a+b)^2 (a+b+1))==2},{a,b}] Out[20]= {{a->-0.4375,b->-0.4375}} Check: In[21]:= a/(a+b)/.sol Out[21]= {0.5} In[22]:= a*b/((a+b)^2 (a+b+1))/.sol Out[22]= {2.} In this case it's better to use NSolve rather than Solve. Tomas > Date: Sat, 25 Jul 2009 04:17:20 -0400 > From: perfreem at gmail.com > Subject: [mg101984] solving a system of two equations > To: mathgroup at smc.vnet.net > > hi all, > > i am trying to find two parameters a, b of the Beta distribution that > make its mean equal to some given constant m and its variance equal to > some given constant v. this reduces to solving a system of two > equations based on the mean/variance definitions of the beta > distribution: > > a/(a+b) = m > a*b/((a + b)^2 (a + b + 1)) = v > > i want to solve this equation for a and b. i tried to solve this in > mathematica, as follows (for m = .5, v = 1): > > Solve[{a/(a + b) == .5, a*b/((a + b)^2 (a + b + 1)) == 2}, a] > But it returns: {} > > i want to get back values for a and b. does anyone know how i can do > this? also, this is subject to the constraint that a and b are > positive real numbers but i am not sure how to express that. > > thank you. >

**References**:**solving a system of two equations***From:*per <perfreem@gmail.com>

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