July 30th, 2018, 08:21 AM  #1 
Newbie Joined: Jul 2018 From: Asia Posts: 5 Thanks: 1  Cosine approximation
how to arrive at (9.11) from (9.10) ? I am stucked during the steps 
July 30th, 2018, 12:42 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,607 Thanks: 616 
$\cos(w_{in}w_{in})t\cos(w_{in}+w_{in})t =\cos^2w_{in}t+\sin^2w_{in}t\cos^2w_{in}t+\sin^2w_{in}t=2\sin^2w_{in}t$ This may help. Last edited by skipjack; August 1st, 2018 at 02:08 PM. 
August 1st, 2018, 05:01 AM  #3 
Newbie Joined: Jul 2018 From: Asia Posts: 5 Thanks: 1 
but the integral of sin() gives cos() ? Do you have any further suggestions ? 
August 1st, 2018, 01:27 PM  #4 
Global Moderator Joined: May 2007 Posts: 6,607 Thanks: 616 
The term with the integral inside the bracket must be small for the approximation to make sense. Can you estimate it?

August 1st, 2018, 02:21 PM  #5 
Global Moderator Joined: Dec 2006 Posts: 19,712 Thanks: 1805 
Is it really an indefinite integral? If so, how come no constant of integration is given?

August 1st, 2018, 06:34 PM  #6 
Newbie Joined: Jul 2018 From: Asia Posts: 5 Thanks: 1  Vm is assumed to be of small magnitude. < How does this help in the approximation towards expression (9.11) ? Last edited by promach; August 1st, 2018 at 06:38 PM. 

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