High school mathematics, solution, what is the formula for calculating the central angle of the side expansion diagram of a circular cone?

1). Preliminary knowledge: cone base radius r, sector radius R, central angle a. Bottom circumference 2兀r = fan-shaped arc length = fan-shaped circle circumference x ratio = 2兀R (a/2兀), about 2兀, r/R=a/2兀. 2). The height of the truncated cone is h, the radius r is small and r is large, and the central angle of the expanded diagram is a radians. Side edge R=root [(r big - r small)^2 h^2]. There is a/2wu=(r big-r small)/R, that is, a=2wu(r big-r small)/R.