Re: Solve's Strange Output

*To*: mathgroup at smc.vnet.net*Subject*: [mg89096] Re: Solve's Strange Output*From*: Szabolcs <szhorvat at gmail.com>*Date*: Sun, 25 May 2008 06:27:02 -0400 (EDT)*References*: <g1avr4$fev$1@smc.vnet.net>

On May 25, 9:09 am, Bruce Colletti <bwcolle... at verizon.net> wrote: > Re 6.0.2 under WinXP. > > This code's output is strange: what does 0.-7.9424 g mean? Ditto for= all values returned by Solve. > > Thankx. > > Bruce > > {x[0],y[0]}={10.5,6.08}; > {x[1],y[1]}={3.23,14.4}; > {x[2],y[2]}={18,12.7}; > m=16.1; > > Solve[{a+c==0,b+d==m*g,d(x[2]-x[0])==c(y[2]-y[0]),a(y[1]-y[0])= ==b(x[1]-x[0])},{a,b,c,d}] > > Out[11]= {{a->0.-7.9424 g,b->0.+9.08951 g,c->0.+7.9424 g,d->0.+7.01049 g= }} It has been mentioned many times that using Solve with inexact numbers invites trouble. Though in this specific case nothing bad happens, it is better to Rationalize the numbers before solving: Solve[Rationalize[{a + c == 0, b + d == m*g, d (x[2] - x[0]) == c (y[2] - y[0]), a (y[1] - y[0]) == b (x[1] - x[0])}], {a, b, c, d}] 0 is not the same as 0.0. The latter is an inexact zero (we only know that it is closer to 0 than $MinMachineNumber), so Mathematica does not simplify 0. + g.

**Follow-Ups**:**Re: Re: Solve's Strange Output***From:*"Szabolcs HorvÃt" <szhorvat@gmail.com>

**Re: Re: Solve's Strange Output***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>