Let the two circles with centre A, B and radii 25cm and 9cm touch each other externally at point C. Then AB=AC+CB=25+9=34cm. Let PQ the direct common tangent. i.e., BQ┴PQ and AP┴PQ. Draw BR┴AP. Then BRQP is a rctangle. (Tangent ┴ radius at point of contact) In ΔABR, AB2=AR2+BR2 (34)2=(16)2+(BR)2 BR2=1156−256=900 BR=√900=30cm Alternatively: Length of common tangent √4r1r2=√4×25×9=30