Given:
\(f(x)=x^3+3x-9\) on \([-2,3]\).
Since \(f'(x)=3x^2+3>0\), \(f\) is increasing, so the greatest value is at \(x=3\):
\(f(3)=27\).
Sum to infinity of decreasing GP:
\(S=\dfrac{a}{1-r}=27\) with
Also \(f'(0)=3\Rightarrow\) difference of first two terms:
\(a-ar=a(1-r)=3\).
From \(a=\;27(1-r)\),
plug into \(a(1-r)=3\): \(27(1-r)^2=3\Rightarrow(1-r)^2=\dfrac{1}{9}\Rightarrow 1-r=\dfrac{1}{3}\) (take positive)
Common ratio: \(r=1-\dfrac{1}{3}=\boxed{\dfrac{2}{3}}\).
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