What substitution should be used to rewrite 4x4 – 21x2 + 20 = 0 as a quadratic equation?

The equation contains x⁴ and x², so let u = x². Then x⁴ = (x²)² = u², and the equation becomes 4u² − 21u + 20 = 0, which is quadratic in u. The correct choice is A, u = x².

AI-written and checked against the verified answer.