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