Flags for trig representation (positive frequency*t+phase)?
08-19-2018, 05:22 AM
Post: #1
 wsprague Junior Member Posts: 9 Joined: Aug 2018
Flags for trig representation (positive frequency*t+phase)?
Hi all,

In a calculation (working through a Laplace Transform problem in a circuits book), I get a result "cos ((3/4)*pi - 2*x)", but I would much prefer something with a positive value for the variable and the phase constant, preferably with the phase to the right instead of the left, like "cos(2*x + BLAH)".

Is there a flag or a trig symbolic rewrite to make this happen?

I tried using "trigcos()" on it but it doesn't change it.

I am using the 2018-07-xx firmware.

(Also "trigcos(cos(-x+pi/2))", just playing around, yields "sin(x)" which seems like the wrong rewrite for the command. Just saying.)

Thanks for the help!
08-19-2018, 02:18 PM
Post: #2
 parisse Senior Member Posts: 1,051 Joined: Dec 2013
RE: Flags for trig representation (positive frequency*t+phase)?
There is no way for the CAS to guess that cos(2x-3/4*pi) is better than cos(3/4*pi-2x). If you want to negate all cos arguments in an expression, you can run subst(expression,cos,x->cos(-x)).
cos(-x+pi/2) is auto-simplified to sin(x), then trigcos will not do anything, because trigcos applies the relation sin(x)^2+cos(x)^2=1 and nothing else (if you run trigcos(cos(-x+pi/2)^2), you will get 1-cos(x)^2).
08-20-2018, 03:32 AM
Post: #3
 wsprague Junior Member Posts: 9 Joined: Aug 2018
RE: Flags for trig representation (positive frequency*t+phase)?
(08-19-2018 02:18 PM)parisse Wrote:  There is no way for the CAS to guess that cos(2x-3/4*pi) is better than cos(3/4*pi-2x). If you want to negate all cos arguments in an expression, you can run subst(expression,cos,x->cos(-x)).
cos(-x+pi/2) is auto-simplified to sin(x), then trigcos will not do anything, because trigcos applies the relation sin(x)^2+cos(x)^2=1 and nothing else (if you run trigcos(cos(-x+pi/2)^2), you will get 1-cos(x)^2).

Pr Parisse -- that is a big help, thank you.

I will keep working on getting *all* sin and cos results to be represented as an equivalent "cos(gamma*t + phi)", but I think that is in the homework stage for me, getting to know the subst() transformation better.
 « Next Oldest | Next Newest »

User(s) browsing this thread: 1 Guest(s)