To simplify the expression \( (2x^4y)(3x^5y^8) \), we can first apply the distributive property and then combine like terms.
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Multiply the coefficients: \[ 2 \cdot 3 = 6 \]
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For the \( x \) terms, use the rule \( x^a \cdot x^b = x^{a+b} \): \[ x^4 \cdot x^5 = x^{4+5} = x^9 \]
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For the \( y \) terms, again use the same rule: \[ y^1 \cdot y^8 = y^{1+8} = y^9 \]
Now put it all together: \[ (2x^4y)(3x^5y^8) = 6x^9y^9 \]
So, the simplified expression is: \[ \boxed{6x^9y^9} \]