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Two circles of radii 1 cm and 4 cm touch each other externally. Find the length of their common (transverse/direct) tangent between the points of contact, i.e. the length of the common tangent.

1. 4 cm
2. 5 cm
3. 6 cm
4. 7 cm

Correct answer: 4 cm

Solution

When two circles touch each other externally, the length of the direct common tangent equals 2*sqrt(r1*r2). Here r1 = 1 cm and r2 = 4 cm, so length = 2*sqrt(1*4) = 2*sqrt(4) = 2*2 = 4 cm.

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