B=centre angle in radians
A=1/2*r^2(B-sinB), numerical solution of B then
H=r-rcos(B/2),
or
B=2arccos(1-H/r)
A=1/2*r^2{2arccos(1-H/r)-sin[2arccos(1-H/r)]}
A=1/2*r^2(B-sinB), numerical solution of B then
H=r-rcos(B/2),
or
B=2arccos(1-H/r)
A=1/2*r^2{2arccos(1-H/r)-sin[2arccos(1-H/r)]}