To simplify the expression \(4a^3b \cdot (3ab)\) using indices, follow these steps:
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Multiply the coefficients: Multiply the numerical coefficients together. \(4 \cdot 3 = 12\).
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Combine the variables using the laws of indices:
- For \(a\): \[ a^3 \cdot a^1 = a^{3+1} = a^4 \]
- For \(b\): \[ b^1 \cdot b^1 = b^{1+1} = b^2 \]
Putting it all together, we have:
\[ 4a^3b \cdot (3ab) = 12a^4b^2 \]
So, the simplified expression is:
\[ \boxed{12a^4b^2} \]