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$求 f(x) = -x^2 + 4x + 9 之極值$

$f'(x)=-2x+4=-2(x-2)$
$\Rightarrow f'(2)=0$
$\Rightarrow f(x)在x=2時可能有極值$

$f'(x)在x<2時大於0，在x>2時小於0$
$\Rightarrow f(x)在x=2時可能有極大值$
$\Rightarrow f(x)之極大值可能為f(2)=13$