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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg104600] Re: Answer for Simplify[Cos[x]^4-Sin[x]^4]?
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Wed, 4 Nov 2009 01:40:23 -0500 (EST)

On 11/3/09 at 2:51 AM, lawrenceteo at yahoo.com (Lawrence Teo) wrote:

>Thanks for the insight. So Simplify[] in Mathematica is right. But
>why I observe small delta if I subtract the two expressions with //
>N? Is it because of machine precision related limitation?

>a = Cos[x]^2 - Sin[x]^2
>b = Cos[x]^4 - Sin[x]^4
>Table[a - b, {x, -10, 10}] // N

>Return small delta...

The issue is the limitations of machine precision numbers. This
is easily demonstrated by

In[36]:= a = Cos[x]^2 - Sin[x]^2;
b = Cos[x]^4 - Sin[x]^4;
Union[Table[a - b, {x, -10, 10}] // Simplify]

Out[38]= {0}

or

In[39]:= Union[Table[a - b, {x, -10, 10}] // N // Chop]

Out[39]= {0}

or most directly by

In[41]:= a - b // Simplify

Out[41]= 0




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