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Correct answer: 5
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We need the number of points where the curve crosses the -axis.
This means we need the number of real roots of
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To determine how many times the graph can cross the -axis, first study the monotonicity using the derivative.
Let . Then
Solving,
Hence the critical points are at
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Let Numerically,
\qquad b\approx 3.33.$$ Since $$f'(x)=5(x^2-(6-\sqrt{26}))(x^2-(6+\sqrt{26})),$$ the sign of $f'(x)$ is: - positive for $|x|<a$ - negative for $a<|x|<b$ - positive for $|x|>b$ Therefore the function behaves as: - increasing on $(-\infty,-b)$ - decreasing on $(-b,-a)$ - increasing on $(-a,a)$ - decreasing on $(a,b)$ - increasing on $(b,\infty)$ -
Now evaluate at convenient points around these intervals to track sign changes.
Since is continuous, each sign change gives a root.
Compute:
So there is one root in .
Next,
So there is one root in .
Next, so between and , there is one root in .
Next,
So there is one root in .
Next,
So there is one root in .
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Thus we have found 5 distinct real roots, in the intervals:
\;(-2,-1), \;(-1,0), \;(1,2), \;(4,5).$$ -
Since a quintic polynomial can have at most 5 real roots, these are all the roots.
Therefore, the number of points where the curve crosses the -axis is
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