noun
common tangent
In geometry, a line that is tangent to two or more curves or surfaces.
二つの円に共通接線を引く。
Draw a common tangent to the two circles.
共通接線の方程式を求めなさい。
Find the equation of the common tangent.
接線 is a tangent to a single curve, while 共通接線 is a tangent shared by two or more curves.
Compound of 共通 (common) + 接線 (tangent).