Three concentric circles have diameters in arithmetic progression. Smallest circumference is 8π cm, largest is 18π
cm. What is the middle circumference?

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Accepted Answer

1. Use the formula $C=\pi d$. So $d=C/\pi$.  
2. Smallest: $C_1=8\pi\Rightarrow d_1=8$. Largest: $C_3=18\pi\Rightarrow d_3=18$.  
3. Diameters are in AP, so middle diameter $d_2=(d_1+d_3)/2=(8+18)/2=13$.  
4. Middle circumference $C_2=\pi d_2=13\pi$. So the correct choice is (c).  
FINAL_ANSWER: c

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Answer

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