GATE 2017 ME (ME1) – Question 3
A right circular cone of base radius 5 cm and height 12 cm is rolled on the ground while its apex P stays fixed, until the marked base point Q touches the ground again. Through what angle in radians about P does it travel?

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Correct answer: (D) $10\pi/13$
Explanation
The slant height is $s=\sqrt{5^2+12^2}=13$ cm. Unrolling the lateral cone surface gives a sector of radius 13 cm and arc length equal to the base circumference $2\pi(5)=10\pi$ cm. The sector angle is arc length divided by radius, $\theta=10\pi/13$ radians. Rolling once until Q returns to the ground sweeps this angle about the fixed apex, giving D.
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