The function $f(x) = x^3 - 6x^2 + 9x + 2$ has maximum value when $x$ is
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To find the local maximum or minimum of a function, first find its critical points by setting the first derivative to zero. Then, use the second derivative test: if $f''(x)<0$ at a critical point, it's a local maximum; if $f''(x)>0$, it's a local minimum.