To begin simplifying the expression \(\frac{(1-2r)(1+2r+4r^2)}{3(1+2r+4r^2)}\), we can observe the common factors in the numerator and the denominator.
The expression has the term \((1+2r+4r^2)\) in both the numerator and the denominator. We can use this common factor to simplify the expression:
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Identify the common factor: \[ 1 + 2r + 4r^2 \]
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Cancel the common factor from both the numerator and the denominator: \[ \frac{(1-2r)\cancel{(1+2r+4r^2)}}{3\cancel{(1+2r+4r^2)}} = \frac{1 - 2r}{3} \]
Thus, the simplified expression is: \[ \frac{1 - 2r}{3} \]