High School

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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 :

We start with the equation

$$
4x^4 - 21x^2 + 20 = 0.
$$

Notice that $x^4$ is the square of $x^2$. This observation suggests making the substitution

$$
u = x^2.
$$

Then, since $x^4 = (x^2)^2$, we can write $x^4 = u^2$. Substituting these into the equation, we obtain

$$
4u^2 - 21u + 20 = 0.
$$

This is now a quadratic equation in $u$. Therefore, the correct substitution is

$$
u = x^2.
$$

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