Question

How would you solve absolute value equations with "or" and "and"? Sometimes even with coefficients.

Answer 1

9514 1404 393

Explanation:

In general, a system of equations has a solution that satisfies all of the equations. That is, the solution set is the intersection (and) of the solution sets of the individual equations in the system. Equations involving the absolute value function are no different.

That being said, you need to consider the meaning of an equation involving the absolute value function.

The function itself is piecewise defined:

  $\displaystyle |x|=\left\{ {x, x\ge0} \atop {-x, x$

So, any equation involving the absolute value function automatically resolves to two equations, each with its own condition on the function value.

__

Example 1:

  |x+1| > 3

is equivalent to the two disjoint conditional equations ...

  (x +1) > 3  and  (x +1) ≥ 0 . . . . OR

  -(x +1) > 3  and  (x +1) < 0

the first of these has the solution x > 2; the second of these has the solution x < -4. The solution set of this equation is the OR of the two solution sets:

  x < -4  or  x > 2

__

Example 2:

  |x +1| < 3

is equivalent to the two disjoint conditional equations ...

  (x +1) < 3  and  (x +1) ≥ 0 . . . . OR

  -(x +1) < 3  and  (x +1) < 0

The first of these has the solution -1 ≤ x < 2, and the second of these has the solution -4 < x < -1. Again, the solution set of this equation is the OR of the two solution sets. However, we find we can write that union as a single compound inequality:

  -4 < x < 2

__

When an absolute value equation involves more than one absolute value function, then the equation will probably resolve into multiple equations, each defined on its own domain. Sometimes keeping those domains straight can be tedious.

Above, we have considered inequalities. If you have an equation, you need to consider that it will likely resolve to two equations. The solution set will be the OR of the solutions to those two equations.

Example 3:

  |x +1| = 3

is equivalent to the two conditional equations ...

  (x +1) = 3  and  (x +1) ≥ 0 . . . . OR

  -(x +1) = 3  and  (x +1) < 0

The first of these has the solution x = 2; the second has the solution x = -4. The solution set is the OR of these two solutions:

  x = -4  or  x = 2

__

The "and" condition restricts the domain of each of the individual pieces of the equation. The "or" condition applies to the collection of solutions that may exist in each of those restricted domains.