Ripetizioni di AnalisiPrecalculusUnlockedITSearch

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 .

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