Question : If C1 and C2 are the centres of two circles and r1 and r2 are the respective radii such that the distance between the centres is equal to the sum of the radii of the two circles, find the number of common tangents.
Option 1: 4
Option 2: 2
Option 3: 1
Option 4: 3
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Correct Answer: 3
Solution : Since the distance between the centres is equal to the sum of the radii of the two circles, the two circles touch each other as shown in the figure. So, there are 3 common tangents. Hence, the correct answer is 3.
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Question : Find the number of common tangents, if $r_1+r_2=c_1 c_2$. (With usual notations, $r_1\ \text{and}\ r_2$ and $\mathrm{C}_1 \ \text{and}\ \mathrm{C}_2$ are the radii and centres of the two circles.)
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