As you now know math mode can be turned on and off several
different ways . The in-text
math mode can be turned on and off using 
and
displayed mode can be turned on and
off by 
or 
.
But both of sometimes we want displayed equations with equation
numbers so we can refer to them later in a paper. The simplest way to do this
for a one line
equation is with the equation environment.
Note that there are two ways to refer to an equation
which does not put parentheses
around the number and 
which does put
parentheses around the number but is only
available if you use the amsmath package. If you did not want to use the amsmath
package
but you wanted a way to get parentheses around a reference number with having to
type
them you could put a statement like 
(which I
did in
this file). Now lets reset the equation counter to zero
Sometimes you would like to have subequations with
counters like (1a) and (1b), etc. This
is d one with the subequations environment.
produces
To make the numbering given in roman numerals, for
example, we can issue a command
like
which al lows us to get
The choices that I know are Alph, alph, Roman, roman,
arabic.
PROBLEM: Write 
commands and words to
display the expression
give it an equation number and then write some text
referring to the equation (e.g., Equation
(blah) is the difference of two squares formula ).
