To simplify the expression \((9x^4y^5)(-2x^2y^7)\), we follow these steps:
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Multiply the coefficients: \(9\) and \(-2\) give \(-18\).
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Multiply the \(x\) terms: \(x^4\) and \(x^2\) can be added together since the bases are the same: \(x^{4+2} = x^6\).
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Multiply the \(y\) terms: \(y^5\) and \(y^7\) can also be added together: \(y^{5+7} = y^{12}\).
Putting it all together, we have:
\[ (9x^4y^5)(-2x^2y^7) = -18x^6y^{12} \]
Therefore, the simplified expression is:
\[ \boxed{-18x^6y^{12}} \]