To simplify the formula \( a = x - 2r^3 \) where \( r = 3x \), let's first substitute the expression for \( r \) into the formula.
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Substitute \( r = 3x \) into \( a = x - 2r^3 \):
\[ a = x - 2(3x)^3 \]
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Calculate \( (3x)^3 \):
\[ (3x)^3 = 27x^3 \]
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Substitute \( 27x^3 \) back into the equation:
\[ a = x - 2(27x^3) \]
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Simplify the equation:
\[ a = x - 54x^3 \]
So, the simplified version of the formula is:
\[ a = x - 54x^3 \]
You mentioned a different expression \( a = x - \frac{x}{3} \). If that is required, please clarify how that expression is to be derived, or if it's part of a different problem. As it stands, the correctly simplified expression based on your initial formula and substitution is \( a = x - 54x^3 \).
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