# Introduction to Equations

An equation containing a variable is neither true nor false until a particular number is substituted for the variable. If a true statement results from such a substitutions, we say that the substitution **satisfies** the equation. For instance, the substitution $$x = 3$$ satisfies the equation $${x^2} = 9$$, but the substitution $$x = 4$$ does not.

An equation that is satisfied by every substitution for which both sides are defined is called an **identity**. For instance, $${\left( {x + 1} \right)^2} = {x^2} + 2x + 1$$ is an identity, as is $${\left( {\sqrt x } \right)^2} = x$$. An equation that is not an identity is called a **conditional equation**. For instance, $$2x = 6$$ is a conditional equation because there is at least one substitution (say, $$x = 4$$) that produces a false statement.

If the substitution $$x = a$$ satisfies an equation, we say that the number $$a$$ is a **solution** or a **root** of the equation. Thus, $$3$$ is a root of the equation $$2x = 6$$, but $$4$$ is not. Two equations are said to be **equivalent** if they have exactly the same roots. Thus, the equation $$2x – 6 = 0$$ is equivalent to the equation $$2x = 6$$ because both equations have one and the same root, $$x = 3$$.

You can change an equation into an equivalent equation by performing any of the following operations:

- Add or subtract the same quantity on both sides of the equation.
- Multiply or divide both sides of the equation by the same nonzero quantity.
- Simplify one or both sides of the equation.
- Interchange the two sides of the equation.

To **solve** an equation means to find all of its roots. The usual method of solving an equation is to write a sequence of equations, starting with the given one, in which each equation is equivalent to the previous one, but “simpler” in some sense. The last equation should either express the solution directly, or be so simple that its solution is obvious.

For example, to solve the equation

\[2x – 6 = 0\]

we begin by adding $$6$$ to both sides to get the equivalent equation.

\[2x = 6\]

Then we divide both sides by $$2$$ to produce the equivalent equation.

\[x = 3\]

The last equation shows that the root is $$3$$.

Variables representing quantities whose value or values we wish to find by solving equations are called **unknowns**. A common practice is to use letters towards the end of the alphabet for unknowns, and letters towards the beginning of the alphabet for **constants** whose values we can assign at will. In particular, the letter $$x$$ is often used for an “unknown quantity,” and the letters $$a$$, $$b$$ and $$c$$ are used for constants. A **literal **or **general equation** is an equation containing, in addition to one or more unknowns, at least one letter that stands for a constant.

For instance,

\[ax + b = 0\]

is a literal equation in which $$x$$is the unknown and the constant coefficients $$a$$ and $$b$$ can be assigned whatever values we please.

If we let $$a = 2$$ and$$b = – 6$$, we obtain

\[2x – 6 = 0\]

whose solution is $$x = 3$$.

In applied mathematics, we cannot always follow the convention that unknowns are represented by letters towards the end of the alphabet, because certain symbols are reserved for special quantities. For instance, in physics $$c$$ is used for the speed of light, $$m$$ is used for mass, and $$v$$ is used for velocity, and so on. We shall specify letters represeningt unknowns to be solved for whenever it isn’t clear from the constant.

An equation such as $$7{x^3} + 3{x^2} + x + 1 = 2x – 5$$, in which both sides are polynomials in the unknown, is called a **polynomial equation**. By subtracting the polynomial on the right from both sides of the polynomial equation, we obtain an equivalent polynomial in **standard form** with zero on the right side.

$$7{x^3} + 3{x^2} + x + 1 = 2x – 5$$

$$7{x^3} + 3{x^2} + x + 1 – \left( {2x – 5} \right) = 0$$ (we subtracted $$2x – 5$$ from both sides)

$$7{x^3} + 3{x^2} – x + 6 = 0$$ (we combined like terms)

The last equation is in standard form. The **degree **of a polynomial equation is defined as the degree of the polynomial on the left side when the equation is in standard form. For instance $$7{x^3} + 3{x^2} + x + 1 = 2x – 5$$ is a third-degree polynomial equation, because after it is written in standard form, $$7{x^3} + 3{x^2} – x + 6 = 0$$, the polynomial on the left side has degree $$3$$.