What is the solution of the following quadratic inequality?x4 – 18x2 ≥ –64

Move −64 to the left: x⁴ − 18x² + 64 ≥ 0. Let y = x²; the quadratic y² − 18y + 64 has roots 9 − √17 ≈ 4.877 and 9 + √17 ≈ 13.123, so it is nonnegative when y ≤ 4.877 or y ≥ 13.123. Since y = x², this gives |x| ≤ 2.208 or |x| ≥ 3.623, with endpoints included. That is choice D.

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