The OP may have used the units palette and document mode. The icon at the top left of the palette is [[unit]], whereas all the others contain a FPS unit eg [[HP]] and [[poundforce/inch^2]]. It appears that entering a unit in the icon at the top left of the palette causes conversion to the default unit system (SI) when Enter is pressed.
It is not necessary to load the Units package - see ?Units,unit and the description under Default Environment
J. Tarr

You need to read the help pages for the Units packages - please see ?Units.
It can be rather overwhelming, so you might like to start by concentrating on the convert units command, please see ?convert,units. If you find the Units package too unwieldy, and some people do, you might like to consider the so-called

Stroud system.
Hope this helps.
J. Tarr

This

site should help you get started.
Good luck.
J. Tarr

You could improve on the above like this:
restart;
ticks := [seq(i=(3500-i),i=0..3500,500)];
plot(x, x = 0..3600, view = [-500..3500,0..3500], tickmarks=[ticks,default], gridlines, axes = box);
Please see ?plot,axis and ?plot,tickmarks.
J. Tarr

You could do something like this:
restart;
eq1:=p*A-(p+dp)*A - rho*g*A*dy = 0;
eq2:=expand(eq1);
eq3 := eq2/dy;
eq4 := simplify(subs(dp=dy*Diff(p(y),y),eq3));
dsolve(eq4);
Hope this helps.
J. Tarr

Remember that you want to substitute in the Left Hand Side of your equation, but not the RHS. So, try this:
restart;
eq := m = m[o]*x;
subs(m=k,lhs(eq))=rhs(eq);
You could do the same thing using eval (and there are Maple gurus who consider that it is better to use eval than subs).
Subscripts can be troublesome in Maple. Please see this

post and this

one.
Good luck.
J. Tarr

You don't even have to download the User Manual - it's part of Maple 11.01. Try Help>Manuals, Dictionary and more>Manuals>User Manual. It's worth taking a look at some of the other items among Manuals, Dictionary and more.
Happy hunting.
J. Tarr

Documents of mixed text and math can be produced in document mode (File>New>Document Mode). Use the F5 key to switch between text and math. Avoid pressing Enter or Ctrl+= while switched to math, and you should be able to produce the kind of document that you want. If you press Enter or Ctrl+=, Maple thinks that you want it to do the math you have just placed in your document e.g. an integration.
You can find more help on document mode at Help>Quick Reference.
Like many others, I dislike document mode, which contains many glitches, and would very much prefer that Maplesoft produced a really good interface with LaTex. That does produce high-class math documents.
J. Tarr

You could get the answer that you want like this:
restart;
ex := c/(a*b)+a*b*d;
eval(ex,a=x/b);
One needs skill and guile to tease the answers out of Maple that you want. They come with time...
J. Tarr

I think that the file mentioned by Scott03 is Zaven Karian's

Statistics Supplement that can be downloaded from the Maple Application Center.
Hope this is useful.
J. Tarr

Reading ?algsubs closely you will find this:
*Note that the requirement for monomials in a to divide monomials in f means that the negative powers of u in the following example are not substituted, and must be handled separately as shown. *
Try this as an extension to the little example that I gave above:
F3 := algsubs(1/u = x^(-1/3),F2);
is(F3=F1);
Hope this de-mystifies the matter.
J. Tarr

The two commands are quite different. Try this:
restart;
f := a/u^4+b/u^2+c+d*u^2+e*u^4;
F1 := eval(f,u= x^(1/3));
F2 := algsubs(u = x^(1/3),f);
is(F1=F2);
Please see ?algsubs and ?eval.
Someone here may be able to tell you how to add algsubs to your favourites palette, but why bother when it is quicker to type the command into your worksheet than doing all that pointy-clicky stuff?
J. Tarr

You seem to be confusing assignment with defining a function. Please see ?:= and ?->
J. Tarr

Try entering ?Si at the command prompt. That will bring up Maple's help page for the Sine, Cosine, etc, integrals. If that overcomes your difficulty, fine. If not, please follow Jacques' advice and upload a worksheet with your problem (click on Upload/Use File or Worksheet Help just above the comment box to discover how to upload a worksheet).
Hope this helps.
J. Tarr

By listing all the assumptions and linearizations, I meant writing them down somewhere as **text** and then using them to build your system equations. Your aim should be to keep the equations as simple as possible.
For example, the resistance to motion of a rotating shaft in a bearing consists of static friction when the motion reverses, Coulomb friction, viscous friction and non-linear (turbulent flow) friction. All of these can be difficult to handle analytically, except viscous friction. So we usually **assume** that that the resistance can be approximated by a torque proportional to the shaft's angular speed. This assumption (which is also a linearization) needs to be stated before writing the equation of motion for the shaft.
Hope this clarifies the matter.
J. Tarr