Answer :
To rewrite the equation
[tex]$$
4x^4 - 21x^2 + 20 = 0
$$[/tex]
as a quadratic, we make the substitution
[tex]$$
u = x^2.
$$[/tex]
Notice that since [tex]$$x^4 = (x^2)^2 = u^2,$$[/tex] the original equation can be rewritten in terms of [tex]$$u$$[/tex] as
[tex]$$
4u^2 - 21u + 20 = 0.
$$[/tex]
This is now a quadratic equation in [tex]$$u$$[/tex]. Therefore, the correct substitution is:
[tex]$$
\boxed{u = x^2.}
$$[/tex]
[tex]$$
4x^4 - 21x^2 + 20 = 0
$$[/tex]
as a quadratic, we make the substitution
[tex]$$
u = x^2.
$$[/tex]
Notice that since [tex]$$x^4 = (x^2)^2 = u^2,$$[/tex] the original equation can be rewritten in terms of [tex]$$u$$[/tex] as
[tex]$$
4u^2 - 21u + 20 = 0.
$$[/tex]
This is now a quadratic equation in [tex]$$u$$[/tex]. Therefore, the correct substitution is:
[tex]$$
\boxed{u = x^2.}
$$[/tex]
