Solve the inequality
Simplify the squared term. An even power destroys the sign: . Writing instead is a frequent slip that changes the whole problem, because it would flip the parabola.
Distribute the -4 across the parentheses. Both inner signs change:
so the inequality becomes
(This shape often arises as the discriminant condition for a quadratic to have two distinct real roots.)
Factor the trinomial. Two numbers multiplying to and adding to are and :
so the inequality is .
Read the sign of the product on each interval. The critical points are and . For both factors are negative, so the product is positive. For the first is positive and the second negative, so the product is negative. For both are positive, so the product is positive again.
Select the intervals where the product is positive, and exclude the roots. Because the inequality is strict, and — where the product is exactly — are not included:
Verify the factorisation and the solution set. The factored form matches the expanded one at sample values of ✓, and scanning points from to confirms the sign pattern agrees with at every point ✓.
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