@acer 

Thanks for the reply.

Here is my worksheet. At the bottom you will see the expressions that I want to simplify.

Example.mw

@acer 

Sorry, you are right. I should have provided more information. The expressions contain cos and sin and up to 14 variables (y1...y14)

Here is an example of an expression which initially had a length of 283197

simplify(expr,size) yields length of 108460

combine(expr) yields length of 7290

simplify(combine(expr),size) yields length of 4482

My goal is to make these expressions as short as possible.

Here is the expression left from simplify(combine(expr),size):

@Preben Alsholm 

Thank you very much, I appreciate your help. Now it works.

I thought convertsys would do that (solving for second derivatives first). Well, maybe it is. But it is taking a lot more time (4hours +). Your solution works fine and takes only a minute.

So thanks again!

@Preben Alsholm 

Thanks for the answer, here is my worksheet (it's about a mechanical problem).

The equations are at the bottom in "Sys", "GenKo" are the variables.

Test.mw

@Preben Alsholm

Thank you very much, your answer helped a lot.

I have one last problem: Currently I want to reduce a (2nd-order) System of 7 ode's (with 7 variables [time-dependent]) to a first-order-system. However, the ode's seem to be "strongly coupled" (is that how you say it? [sorry, english is not my native language]), so Maple has trouble seperating the variables...So far my program has run 4 hours, but to no avail.

Would you know how to tackle this? Or is this as far as it goes?

P.S.: I know there is a method involving a "mass-matrix": M*zp=F(t,z) but I couldn't find anything like that for Maple...

@Preben Alsholm 

Thanks for the answer. I still have some questions though.

How is a(t) a "known" function? I thought these were known functions:

http://www.maplesoft.com/support/help/Maple/view.aspx?path=initialfunctions

The ddiff-workaround is not an option since I have multiple equations that need to be reduced to a first order system.

From what I experienced now I can use complete gibberish instead of "a" as long as it doesn't begin with a,b,c,d,e. Anything else seems to be working.

Guess I'll just rename my variables...

Still I'm curious as to why Maple acts like that....