Output of symoblic expression coming out incorrectly

Hi guys,

I am using the code pasted below with the corresponding output:

syms X w t p

eq = X*cos(w*t - p)

eq =

X*cos(p - t*w)

The output is is reversed it should instead be X*cos(w*t - p). How would I change this?

[cocopuffs]

Roger Stafford is correct. I forgot cos had that property. I graphed my solutions using both configurations and got the same values.

I was thinking about the sin function because the order actually matters. Thank you for clearing that up.

Answer 1

[Roger Stafford]

The two expressions are identically equal for the cosine function, just as t*w and w*t are identically equal because of the commutative law of multiplication. The symbolic toolbox has a tendency to rearrange expressions in accordance with some criteria unknown to us mere mortals, provided the result is still identically equal.

[Star Strider]

One clue is in the documentation for Find a Default Symbolic Variable:

‘If you do not specify an independent variable when performing substitution, differentiation, or integration, MATLAB uses a default variable. The default variable is typically the one closest alphabetically to x or, for symbolic functions, the first input argument of a function. To find which variable is chosen as a default variable, use the symvar(f, 1) command.’

I’m not sure this demystifies, but it helps explain some of the occasionally arcane choices the Symbolic Toolbox makes.

[Roger Stafford]

Somehow I doubt that a default symbolic variable was involved in this particular instance. It looks like the typical rearrangement that often occurs to symbolic expressions due to - well, call it "simplification" - in this case it put the variables in alphabetical order, but nevertheless following specific rules of equivalence. My own ancient copy of the Symbolic Toolbox often does such peculiar things.