弧长C=390拱高H=40，怎么算弧直径D？弧半径为R。Rn+1=(1-(Rn*COS(C/(2*Rn))-Rn+H)/((C/2)*SIN(C/(2*Rn))-H))*Rn R0=475R1=468.31R2=468.49R3=468.49R=468.49D=2*D=2*468.49=936.98
缺少条件，还要有，弦长或拱高
