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trig

 Apr 25, 2019
edited by kawaiiai  Apr 25, 2019
 #1
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Evaluating each

 

cos ( x + y) - cos (x - y)  =   [ cosxcosy - sinxsin y] - [ cosxcosy + sinxsiny ]  = -2sinxsiny

Not an identity

 

sin(x + y) - sin (x - y)  =  [sinxcosy + sinycosx] - [ sinxcosy - sin ycosx ]  =  2cosxsiny

Identity

 

The third is not an identity  [ see the first one ]

 

sin (x - pi)  =  sinxcospi - sin pi cosx  =   -1sin x  - 0  =  -sin x

Identity

 

 

cool cool cool

 Apr 25, 2019

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