已知弧长C，弦长L，求弓形面积S?弧半径为R，弧所对的圆心角为A。用下式来求得RRn+1=(1+(L-2*Rn*SIN(C/(2*Rn)))/(L-C*COS(C/(2*Rn))))*RnA=C/RS=π*R^2*A/(2*π)=R^2*A/2
