That is, a= central angle *π/ 180.
For a sector, l = a * r.
s fan = 1/2l 1r 1 = 1/2 * A * r 1 2
S small fan =1/2l2r2 =1/2 * a * r22.
S shadow =S big fan -S small fan = 1/2 a * (r 1 2-R2 2)
= 1/2a *(r 1+R2)(r 1-R2)
= 1/2(l 1+L2)(r 1-R2)
= 1/2(L 1+L2)d