Question

What substitution should be used to rewrite 4x4 – 21x2 + 20 = 0 as a quadratic equation?
u = x2
u = 2x2
u = x4
u = 4x4

Answer 1

Let u = x^2. Then x^4 = u^2, so the equation becomes 4u^2 − 21u + 20 = 0, which is quadratic in u.