To find the greatest common factor (GCF) of the terms \(12a^2\) and \(20ab\), we start by factoring each term.
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Factor 12 and 20:
- The prime factorization of 12 is \(2^2 \times 3\).
- The prime factorization of 20 is \(2^2 \times 5\).
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Identify the common factors of the coefficients:
- The common factor of \(12\) and \(20\) is \(2^2\), which is \(4\).
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Factor the variables:
- For \(a^2\) from \(12a^2\) and \(ab\) from \(20ab\), the common factor is \(a\) (since it has the lowest exponent).
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Combine the common factors:
- The GCF of the coefficients is \(4\) and the GCF of the variable parts is \(a\).
- Therefore, the GCF of \(12a^2\) and \(20ab\) is \(4a\).
Thus, the greatest common factor is 4a.