Up until now, we have just been talking about
manipulating algebraic expressions. Now it is time to talk about *equations*. An expression is just a
statement like

2*x* + 3

This expression might be equal to any number, depending on the choice of *x*.
For example, if *x* = 3 then the value of this expression is 9.
But if we are writing an equation, then we are making a statement about its
value. We might say

2*x* + 3 = 7

A mathematical equation is either true or false. This equation, 2*x* + 3 = 7,
might be true or it might be false; it depends on the value chosen for *x*.
We call such equations *conditional*, because their truth depends on
choosing the correct value for *x*. If I choose *x* = 3,
then the equation is clearly false because 2(3) + 3 = 9,
not 7. In fact, it is only true if I choose *x* = 2. Any other
value for *x* produces a false equation. We say that *x* = 2
is the *solution* of this equation.

- The solution of an
equation is the value(s) of the variable(s) that make the equation a true
statement.

An equation like 2*x* + 3 = 7 is a simple type
called a linear equation in one variable. These will always have one solution,
no solutions, or an infinite number of solutions. There are other types of
equations, however, that can have several solutions. For example, the equation

*x*^{2} = 9

is satisfied by both 3 and –3, and so it has two solutions.

This is the normal case, as in our example where the equation 2*x* + 3 = 7
had exactly one solution, namely *x* = 2. The other two cases,
no solution and an infinite number of solutions, are the oddball cases that you
don’t expect to run into very often. Nevertheless, it is important to know that
they can happen in case you do encounter one of these situations.

Consider the equation

*x* = *x*

This equation is obviously
true for every possible value of *x*. This is, of course, a ridiculously
simple example, but it makes the point. Equations that have this property are
called *identities*. Some examples of identities would be

2*x* = *x* + *x*

3 = 3

(*x *– 2)(*x* + 2) = *x*^{2}
– 4

All of these equations are true for any value
of *x*. The second example, 3 = 3, is interesting because it
does not even contain an* x*, so obviously its truthfulness cannot depend
on the value of *x*! When you are attempting to solve an equation
algebraically and you end up with an obvious identity (like 3 = 3),
then you know that the original equation must also be an identity, and
therefore it has an infinite number of solutions.

Now consider the equation

*x* + 4 = *x* + 3

There is no possible value
for *x* that could make this true. If you take a number and add 4 to it,
it will never be the same as if you take the same number and add 3 to it. Such
an equation is called a *contradiction*, because it cannot ever be true.

If you are attempting to solve such an equation, you will
end up with an extremely obvious contradiction such as 1 = 2. This
indicates that the original equation is a contradiction, and has no solution.

In summary,

o
An *identity* is always
true, no matter what *x* is

o
A *contradiction* is
never true for any value of *x*

o
A *conditional equation* is true for some values of *x*