Worked examples · Sheet 3
Inequalities of the form |A| < k and |A| ≤ k
Theory used: Inequality of the form |A| < kAbsolute value as distance
Go to: Exercise 1Exercise 2Exercise 3
Exercise 1
Solve the inequality
Since , the inequality is equivalent to the system
The solution is .
The right-hand side is the positive constant , so the inequality is equivalent to the double inequality , written as the system of the two conditions. Adding to all sides gives the interval . The geometric reading confirms the result, since asks for the points at distance less than from , and these form the open interval with endpoints and .
Exercise 2
Solve the inequality
The solution is .
The inequality is non-strict, so the two conditions of the system are also non-strict and the endpoints of the interval belong to the solution. In each condition the is moved to the right and we divide by , which is positive, therefore the direction of the inequalities does not change. Checking at the endpoints confirms the result: for we have , and for we have ; for an exterior point such as , instead, we have .
Exercise 3
Solve the inequality
The solution is .
The argument of the absolute value is quadratic, but the rule is the same: the inequality is equivalent to the system of the two conditions and . The condition is satisfied on the interval , while the condition is satisfied outside the open interval with endpoints and . The two conditions must hold together, so we intersect the interval with the union of half-lines, and the two symmetric intervals shown remain. Checking with a test value confirms the result: for we have and , while for we have .