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Re: Different answers in mathematica and my calculator.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg125647] Re: Different answers in mathematica and my calculator.
  • From: Dana DeLouis <dana01 at me.com>
  • Date: Sun, 25 Mar 2012 00:16:02 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

> 4/(3Power[2(-2)+3, (3)^-1])
> 
> I get -(4/3) (-1)^(2/3) in Mathematica but only -4/3 on my calculator.


Hi.  Depending on the math one is doing, this general idea might be a possible solution.

fix  = (x_)?Negative^
     (Rational[a_, b_]) :> 
   Piecewise[{{(x^a)^(1/b), 
      EvenQ[a]}, {-((-x)^a)^(1/b), 
      OddQ[b]}, {x^(a/b), True}}];


Here are 3 numbers:

equ={
	4/(3Power[2(-2)+3,(3)^-1]) ,
	Power[-8,2/3],
	Power[-27,1/3]  }

{-(4/3) (-1)^(2/3),  4 (-1)^(2/3),  3 (-1)^(1/3)}

For something quick-n-dirty,  this makes an attempt for what you want.

equ /. fix
{-(4/3), 4, -3}

For complex numbers, the magnitude is the value, but as you can see, the sign can be wrong.
For some things, that's all one might need thou.

Abs /@ equ
{4/3, 4, 3}

= = = = = = = = = = = =
HTH  :>)
Dana DeLouis
Mac & Math 8
= = = = = = = = = = = =



On Mar 19, 6:04 am, Nile <thrasher... at gmail.com> wrote:
> 4/(3Power[2(-2)+3, (3)^-1])
> 
> I get -(4/3) (-1)^(2/3) in Mathematica but only -4/3 on my calculator.
> 
> N[Sec[8/(8 Sqrt[2])]]/Degree to get Cos^-1 of 8/(8 Sqrt[2]) and it gives me 75 deg instead of 45...
> 
> I'm not sure what I'm doing wrong, I tried in Wolfram Alpha and it gives me the same thing.
> 
> Thank you.
> 
> -Francis





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