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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 perform a substitution that transforms the equation into a standard quadratic form.

Here’s how we can do it:

1. Identify terms involving [tex]\(x\)[/tex]: Notice that this equation involves powers of [tex]\(x^4\)[/tex] and [tex]\(x^2\)[/tex]. Specifically, [tex]\(x^4\)[/tex] can be expressed as [tex]\((x^2)^2\)[/tex].

2. Choose an appropriate substitution: To simplify the equation to a quadratic form, let’s set [tex]\(u = x^2\)[/tex]. This substitution turns the term [tex]\(x^4\)[/tex] into [tex]\(u^2\)[/tex] because [tex]\((x^2)^2 = u^2\)[/tex].

3. Substitute and simplify: Replace all occurrences of [tex]\(x^2\)[/tex] with [tex]\(u\)[/tex] in the original equation. The equation becomes:

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

Substitute [tex]\(x^2\)[/tex] with [tex]\(u\)[/tex]:

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

Now, the original equation is rewritten as the quadratic equation [tex]\(4u^2 - 21u + 20 = 0\)[/tex] in terms of [tex]\(u\)[/tex].

Therefore, the correct substitution to use for rewriting the given equation as a quadratic one is [tex]\(u = x^2\)[/tex].

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