rootsearch in a piecewise function
- To: mathgroup at smc.vnet.net
- Subject: [mg60247] rootsearch in a piecewise function
- From: juejung <juejung at indiana.edu>
- Date: Thu, 8 Sep 2005 04:53:39 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
hi group,
why does the root search in the following piecewise function not work,
when the plot function before evaluates just fine.
if i use the /.para already at the point where i define f2 and f3 then the
root command works. however, the actual functions f2 and f3 are much
longer and i would like to replace parametervalues only at the point where
i do in the example below. it seems that findroot is evaluated before the
replacement takes place??
thanks
juergen
para = {a -> 3, b -> 4, c -> 20, d -> 1.5};
f2[x_] := 10 + a* x^(1/2) - x - b;
f3[x_] := -6 + c/x + d^2;
f1[x_] := Which[0 < x < b, f2[x], b <= x < 10, f3[x],10 <= x, x^(1/2)]
/.para;
df1[x_] = D[f1[x], x];
Plot[f1[x] /.para, {x, 0.1, 15}]
Plot[df1[x] /.para, {x,0.1, 15}]
FindRoot[{f1[x] == 0} /.para, {x, 2}]
FindRoot[{df1[x] == 0} /.para, {x, 2}]