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

*To*: mathgroup at smc.vnet.net*Subject*: [mg129889] Re: ComplexExpand off by a factor of (-1)?*From*: Bob Hanlon <hanlonr357 at gmail.com>*Date*: Sat, 23 Feb 2013 07:01:26 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <20130222060524.703B768D0@smc.vnet.net>

expr = (E^(I*y) - E^(-I*y))/2*I; If you plot your expression it is clear that it is -Sin[y] rather than Sin[y] Plot[expr, {y, -Pi, Pi}] ComplexExpand[expr] -Sin[y] Term-by-term comparison in power series Assuming[{Element[n, Integers]}, SeriesCoefficient[expr, {y, 0, n}] == SeriesCoefficient[-Sin[y], {y, 0, n}] // FullSimplify] True Bob Hanlon On Fri, Feb 22, 2013 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]? > > Thanks. > > Charles S. >

**References**:**ComplexExpand off by a factor of (-1)?***From:*charles.ces@gmail.com