To factor the expression \( 18x^4 - 12x^2 \) completely, we can follow these steps:
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Identify the Greatest Common Factor (GCF):
- The coefficients \( 18 \) and \( 12 \) have a GCF of \( 6 \).
- The variable terms \( x^4 \) and \( x^2 \) have a GCF of \( x^2 \).
Therefore, the GCF of the entire expression is \( 6x^2 \).
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Factor out the GCF: \[ 18x^4 - 12x^2 = 6x^2(3x^2 - 2) \]
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Check if the remaining expression can be factored:
- The expression \( 3x^2 - 2 \) is a difference of terms, but it does not factor nicely over the integers or rationals as it is irreducible.
Thus, the completely factored form of the expression \( 18x^4 - 12x^2 \) is:
\[ \boxed{6x^2(3x^2 - 2)} \]