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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 rewrite the equation [tex]\(4x^4 - 21x^2 + 20 = 0\)[/tex] as a quadratic equation, we need to use a substitution that simplifies the equation into a standard quadratic form.

1. Identify the structure of the polynomial:
Notice that the polynomial includes terms with [tex]\(x^4\)[/tex] and [tex]\(x^2\)[/tex]. Specifically, [tex]\(x^4\)[/tex] is a power that is twice the power of [tex]\(x^2\)[/tex].

2. Choose an appropriate substitution:
We can use the substitution [tex]\(u = x^2\)[/tex]. This is a common technique when dealing with equations where one exponent is twice the other, like [tex]\(x^4\)[/tex] and [tex]\(x^2\)[/tex].

3. Rewrite each term using the substitution:
- The term [tex]\(x^4\)[/tex] can be rewritten as [tex]\((x^2)^2\)[/tex], which, using our substitution [tex]\(u = x^2\)[/tex], becomes [tex]\(u^2\)[/tex].
- The term [tex]\(x^2\)[/tex] directly becomes [tex]\(u\)[/tex].

4. Substitute into the equation:

Original equation: [tex]\(4x^4 - 21x^2 + 20 = 0\)[/tex]

Applying the substitution, we get:
[tex]\[
4(u^2) - 21u + 20 = 0
\][/tex]

This is a quadratic equation in terms of [tex]\(u\)[/tex].

Thus, the substitution [tex]\(u = x^2\)[/tex] should be used to rewrite the equation as a quadratic in [tex]\(u\)[/tex].

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