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2001 AMC 12 Problems/Problem 8

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Problem

Which of the cones listed below can be formed from a 252^\circ sector of a circle of radius 10 by aligning the two straight sides?

import graph;unitsize(1.5cm);defaultpen(fontsize(8pt));draw(Arc((0,0),1,-72,180),linewidth(.8pt));draw(dir(288)--(0,0)--(-1,0...

\text{(A) A cone with slant height of } 10 \text{ and radius } 6

\text{(B) A cone with height of } 10 \text{ and radius } 6

\text{(C) A cone with slant height of } 10 \text{ and radius } 7

\text{(D) A cone with height of } 10 \text{ and radius } 7

\text{(E) A cone with slant height of } 10 \text{ and radius } 8

Solution

import graph;unitsize(1.5cm);defaultpen(fontsize(8pt));draw(Arc((0,0),1,-72,180),linewidth(.8pt) + red);draw(dir(288)--(0,0)-...

The blue lines will be joined together to form a single blue line on the surface of the cone, hence \boxed{10} will be the \boxed{\text{slant height}} of the cone.

The red line will form the circumference of the base. We can compute its length and use it to determine the radius.

The length of the red line is \dfrac{252}{360}\cdot 2\pi \cdot 10 = 14\pi. This is the circumference of a circle with radius \boxed{7}.

Therefore the correct answer is \boxed{\text{C}}.

See Also

2001 AMC 12 (ProblemsResources)
Preceded by
Problem 7
Followed by
Problem 9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Looking for a challenging algebra text? Preparing for MATHCOUNTS or the AMC exams?
Check out Art of Problem Solving's Introduction to Algebra by Richard Rusczyk.
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