Two circles of radii 9 cm and 4 cm touch each other externally. Find the length of their direct common tangent.

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Accepted Answer

1. The centers are at distance $d=r_1+r_2=9+4=13$.  
2. The direct common tangent length is $L=\sqrt{d^2-(r_1-r_2)^2}$.  
3. So $L=\sqrt{13^2-5^2}=\sqrt{169-25}=\sqrt{144}=12$.  
4. The tangent length is 12 cm, so the correct choice is **(b)**.

FINAL_ANSWER: b

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