Where is substitution should be used to rewrite 4x^4-21x^2+20=0

u = x²

Step-by-step explanation:

4x⁴ - 21x² + 20 = 0

Substitute u = x² to turn the equation into a quadratic.

4u² - 21u + 20 = 0

You can solve by factoring.

(4u - 5) (u - 4) = 0

u = 5/4 or 4

x² = 5/4 or 4

x = ±½√5 or ±2