Find all real numbers x such that x^4 - 5x^2 + 4 = 0.
Correct: A
Let y = x^2. Then the equation becomes y^2 - 5y + 4 = 0. Factoring, we have (y-1)(y-4) = 0. So y = 1 or y = 4. If y = 1, then x^2 = 1, so x = ±1. If y = 4, then x^2 = 4, so x = ±2. Thus, the solutions are x = -2, -1, 1, 2.