The distance between the centres of two circles is 81 cm, and their radii are 42 cm and 51 cm. What is the length (in cm) of the direct common tangent to the circles?

Correct option is B

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Given:
Radii of two circles as $$r_1$$​ = 51cm and $$r_2$$​ = 42cm.
Distance between the center{d} = 81cm.
Formula Used:
Direct common tangent = $$\sqrt{d^2 - (r_1 - r_2)^2}$$​​
Solution:
Direct common tangent = $$\sqrt{81^2 - (51 - 42)^2}$$​​

​$$= \sqrt{6561 - 9^2}\\= \sqrt{6561 - 81}\\= \sqrt{6480} = 36\sqrt{5} \, \text{cm}\\$$​​