The Maclaurin series for $\cosh x$ is $1 + x^2/2! + x^4/4! + x^6/6! + \cdots$. We substitute $3x$ for $x$ to get the Maclaurin series for $\cosh 3x$. This gives us $1 + (3x)^2/2! + (3x)^4/4! + (3x)^6/6! + \cdots$. Simplifying, we get $1 + 9x^2/2! + 81x^4/4! +
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