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What substitution should be used to rewrite [tex]$4x^4 - 21x^2 + 20 = 0$[/tex] as a quadratic equation?

A. [tex]u = x^2[/tex]
B. [tex]u = 2x^2[/tex]
C. [tex]u = x^4[/tex]
D. [tex]u = 4x^4[/tex]

Answer :

To transform the equation

[tex]$$
4x^4 - 21x^2 + 20 = 0
$$[/tex]

into a quadratic form, we use a substitution that simplifies the quartic term. Notice that the equation involves [tex]$x^4$[/tex] and [tex]$x^2$[/tex], and [tex]$x^4$[/tex] can be written as [tex]$(x^2)^2$[/tex]. This suggests using the substitution

[tex]$$
u = x^2.
$$[/tex]

Now, substitute [tex]$u$[/tex] into the equation:

1. Replace [tex]$x^4$[/tex] with [tex]$u^2$[/tex] because [tex]$x^4 = (x^2)^2 = u^2$[/tex].
2. Replace [tex]$x^2$[/tex] with [tex]$u$[/tex].

The equation becomes

[tex]$$
4u^2 - 21u + 20 = 0.
$$[/tex]

This is now a standard quadratic equation in [tex]$u$[/tex]. Therefore, the correct substitution is

[tex]$$
u = x^2.
$$[/tex]

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