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Solve the inequality x⁴ - 5x² + 4 < 0 over the real numbers.

1. (-2, -1) union (1, 2)
2. (-1, 1)
3. (-2, 2)
4. (-inf, -2) union (2, inf)

Correct answer: (-2, -1) union (1, 2)

Solution

Factor as (x² - 1)(x² - 4) = (x-1)(x+1)(x-2)(x+2). The product is negative between consecutive roots. With roots at -2, -1, 1, 2, the expression is negative on (-2, -1) and (1, 2).

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