Two circles with radii 'a' and 'b' respectively touch each other externally. Let 'c' be the radius of a circle that touches these two circles as well as a common tangent to the two circles. Then
a. −1/√a−1/√b=1/√c
b. 1/√a−1/√b=−1/√c
c. 1/√a+1/√b=1/√c
d. 1/√a−1/√b=1/√c