Putting Expressions into Different Forms
Complicated algebraic expressions can usually be written in many different ways.
Mathematica provides a variety of functions for converting expressions from one form to another.
In many applications, the most common of these functions are
Expand,
Factor and
Simplify. However, particularly when you have rational expressions that contain quotients, you may need to use other functions.
Expand[expr]  multiply out products and powers 
ExpandAll[expr]  apply Expand everywhere 
Factor[expr]  reduce to a product of factors 
Together[expr]  put all terms over a common denominator 
Apart[expr]  separate into terms with simple denominators 
Cancel[expr]  cancel common factors between numerators and denominators 
Simplify[expr]  try a sequence of algebraic transformations and give the smallest form of expr found 
Functions for transforming algebraic expressions.
Here is a rational expression that can be written in many different forms.
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Expand expands out the numerator, but leaves the denominator in factored form.
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Together collects all the terms together over a common denominator.
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Apart breaks the expression apart into terms with simple denominators.
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Factor factors everything, in this case reproducing the original form.
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According to Simplify, this is the simplest way to write the original expression.
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Getting expressions into the form you want is something of an art. In most cases, it is best simply to experiment, trying different transformations until you get what you want. Often you will be able to use palettes in the front end to do this.
When you have an expression with a single variable, you can choose to write it as a sum of terms, a product, and so on. If you have an expression with several variables, there is an even wider selection of possible forms. You can, for example, choose to group terms in the expression so that one or another of the variables is "dominant".
Collect[expr,x]  group together powers of x 
FactorTerms[expr,x]  pull out factors that do not depend on x 
Rearranging expressions in several variables.
Here is an algebraic expression in two variables.
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This groups together terms in v that involve the same power of x.
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This groups together powers of y.
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This factors out the piece that does not depend on y.
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As we have seen, even when you restrict yourself to polynomials and rational expressions, there are many different ways to write any particular expression. If you consider more complicated expressions, involving, for example, higher mathematical functions, the variety of possible forms becomes still greater. As a result, it is totally infeasible to have a specific function built into
Mathematica to produce each possible form. Rather,
Mathematica allows you to construct arbitrary sets of transformation rules for converting between different forms. Many
Mathematica packages include such rules; the details of how to construct them for yourself are given in
"Transformation Rules and Definitions".
There are nevertheless a few additional builtin
Mathematica functions for transforming expressions.
TrigExpand[expr]  expand out trigonometric expressions into a sum of terms 
TrigFactor[expr]  factor trigonometric expressions into products of terms 
TrigReduce[expr]  reduce trigonometric expressions using multiple angles 
TrigToExp[expr]  convert trigonometric functions to exponentials 
ExpToTrig[expr]  convert exponentials to trigonometric functions 
FunctionExpand[expr]  expand out special and other functions 
ComplexExpand[expr]  perform expansions assuming that all variables are real 
PowerExpand[expr]  transform (xy)^{p} into x^{p}y^{p}, etc. 
Some other functions for transforming expressions.
This expands out the trigonometric expression, writing it so that all functions have argument x.
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This uses trigonometric identities to generate a factored form of the expression.
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This reduces the expression by using multiple angles.
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This expands the sine assuming that x and y are both real.
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This does the expansion allowing x and y to be complex.
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The transformations on expressions done by functions like
Expand and
Factor are always correct, whatever values the symbolic variables in the expressions may have. Sometimes, however, it is useful to perform transformations that are only correct for some possible values of symbolic variables. One such transformation is performed by
PowerExpand.
Mathematica does not automatically expand out noninteger powers of products.
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