RE: How do I pick out the expression under a radical?
- To: mathgroup at smc.vnet.net
- Subject: [mg36247] RE: [mg36225] How do I pick out the expression under a radical?
- From: "David Park" <djmp at earthlink.net>
- Date: Wed, 28 Aug 2002 04:16:07 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Steve,
Here is a slightly more complicated case.
expr = a b x^2 + 5 x^3 + 5 Sqrt[4 - x^2] - 1/Sqrt[5 - 2x^2]
The following picks out the expressions under square roots. Square roots are
represented as Power[a,1/2] and if they are in the denominator they are
a -1/2 power. We use an Alternative in the pattern to pick out both.
rexprs = Cases[expr, Power[a_, 1/2 | -1/2] -> a, Infinity]
{5 - 2*x^2, 4 - x^2}
The following Standard Package is useful.
Needs["Algebra`InequalitySolve`"]
And @@ (# >= 0 & /@ rexprs)
InequalitySolve[%, x]
5 - 2*x^2 >= 0 && 4 - x^2 >= 0
-Sqrt[5/2] <= x <= Sqrt[5/2]
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
From: Steve Beach [mailto:asb4 at psu.edu]
To: mathgroup at smc.vnet.net
I'm a poor physicist trying to figure out how to sort out the
"physical" from the "non-physical" solutions to a problem. To do
that, I need to be able to look at an expression and pick out a
subexpression, the part under the radical.
For example, say I've got the expression
a b x^2 + 5 x^3 + 5 Sqrt[4 - x^2]
I'd like to pick out "4 - x^2", which would then tell me that x is
between +/- 2. I know there has got to be an easy way to do it, but I
can't find it. Any help would be appreciated.
Thanks,
Steve Beach
asb4 at psu.edu
http://www.thebeachfamily.org