What substitution should be used to rewrite 4x^4 + 21x^2 + 20 = 0 as a quadratic equation?

To rewrite the equation 4x⁴ + 21x² + 20 = 0 as a quadratic equation, we can make a substitution that simplifies the expression. In this case, we can let y = x². This means that x⁴ = (x²)² = y².

With this substitution, our original equation transforms as follows:

• 4x⁴ + 21x² + 20 becomes 4y² + 21y + 20 = 0.

This is now a standard quadratic equation in terms of y. It can be solved using the quadratic formula or by factoring, making it much easier to work with.