Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2009

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Answer for Simplify[Cos[x]^4-Sin[x]^4]?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg104450] Re: Answer for Simplify[Cos[x]^4-Sin[x]^4]?
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Sun, 1 Nov 2009 03:57:38 -0500 (EST)

On 10/30/09 at 2:19 AM, lawrenceteo at yahoo.com (Lawrence Teo) wrote:

>We know that Simplify[Cos[x]^2-Sin[x]^2] -> Cos[2 x] But why
>Simplify[Cos[x]^4-Sin[x]^4] -> Cos[2 x] too?

>Doing subtraction between the two expressions will give small delta.
>This is enough to prove that the two expression shouldn't be the
>same.

When you did the subtraction did you do:

In[5]:= Cos[x]^2 - Sin[x]^2 - (Cos[x]^4 - Sin[x]^4) // Simplify

Out[5]= 0

which is correct. Or did you fail to include the parentheses and do

In[6]:= Cos[x]^2 - Sin[x]^2 - Cos[x]^4 - Sin[x]^4 // Simplify

Out[6]= -2 sin^4(x)

Since, this result could be characterized as a "small delta", my
guess is this is what you did. But this is not the difference
between the two expressions.




  • Prev by Date: dynamicmodule with f[x_] possible?
  • Next by Date: SetDelayed and Evaluate
  • Previous by thread: Re: Re: dynamicmodule with f[x_] possible?
  • Next by thread: Re: Answer for Simplify[Cos[x]^4-Sin[x]^4]?