- A
- B
- C
- D
View written solutionFree
Correct answer: C
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Interpret the curves
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Point lies on the circle which has center and radius .
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Point lies on the parabola
We need the point on the circle for which the distance to the parabola is minimum.
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Geometric idea for minimum distance between two smooth curves
If the minimum distance occurs at points and on the two curves, then the segment is along the common normal to both curves.
Also, for a circle, the normal at is along the radius . Hence at the minimizing position, the line joining to must pass through the center .
Therefore, if is the corresponding point on the parabola, then the normal to the parabola at must pass through .
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Equation of normal to the parabola
Let on .
Slope of tangent at is
So slope of normal is
Equation of the normal at is
Since this normal passes through , substitute , :
Multiply by :
Hence,
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Solve the cubic
We test :
=\frac14+\frac32-1 =\frac34\neq 0. $$ Test $t=\frac13$: $$ 2\left(\frac{1}{27}\right)+3\left(\frac13\right)-1 =\frac{2}{27}+1-1 =\frac{2}{27}\neq 0. $$ Test $t\approx 0.3$: $$ 2(0.027)+0.9-1=-0.046. $$ Test $t\approx 0.32$: $$ 2(0.032768)+0.96-1=0.025536. $$ So the root lies near $t\approx 0.31$. We now find the corresponding point $P$ on the circle.
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Find from the center toward
Since is on the circle and lies on the line from center toward , we have
Here
The abscissa of is
Using :
Then
\approx \sqrt{0.4761+1.2014} \approx \sqrt{1.6775} \approx 1.295. $$ Therefore, $$ x_P\approx 1-\frac{0.69}{1.295} \approx 1-0.533 \approx 0.467. $$ Thus the abscissa of $P$ lies in $$ \left(\frac14,\frac12\right). $$
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Check options
- A: ❌
- B: ❌
- C: ✅
- D: ❌
So the correct option is C.
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Comparison with stored answer
Stored correct answer: C
Our derived answer: C
Hence, they agree.
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