先求出弧半径R,设弦长为a,弧高h ,根据勾股定理,
R^2=(R-h)^2+(a/2)^2,
R^2=R^2-2Rh+h^2+a^2/4,
R=(0.8^2+3^2)/(2*0.8)=6.025(m),
再求圆心角度数α,
tan(α/2)=(a/2)/(R-h)=3/5.225=0.5741,
α/2=29°53'=29.88°,
α=59.76°
∴弧长s=nπR/180=59.76*3.1416*6.025/180=6.284(m).
近似是60度扇形,2πR/6≈2π.