What substitution should be used to rewrite 4 x to the 4th power minus 21 x squared plus 20 is equal to 0 as a quadratic equation?

The equation contains both x⁴ and x², and x⁴ = (x²)². Let u = x², so the equation becomes 4u² − 21u + 20 = 0, which is quadratic in u. Therefore, the correct choice is “u is equal to x squared.”

AI-written and checked against the verified answer.