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Re: ComplexExpand off by a factor of (-1)?

• To: mathgroup at smc.vnet.net
• Subject: [mg129887] Re: ComplexExpand off by a factor of (-1)?
• From: Bill Rowe <readnews at sbcglobal.net>
• Date: Sat, 23 Feb 2013 07:00:46 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• Delivered-to: l-mathgroup@wolfram.com
• Delivered-to: mathgroup-newout@smc.vnet.net
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On 2/22/13 at 1:05 AM, charles.ces at gmail.com wrote:

>Would someone be able to help me understand why when I enter the
>expression:

>ComplexExpand[(E^(I*y) - E^(-I*y))/2*I] I get a result of -Sin[y]
>instead of Sin[y]?

You get -Sin[y] because that is the correct answer. You probably intended

In[4]:= ComplexExpand[(E^(I*y) - E^(-I*y))/(2*I)]

Out[4]= Sin[y]

that is divide by 2 I rather than divide by 2 and *multiply* by
I as you did. And since 1/I = -1 you got the correct result