六米八的跨度,弧起一米四的高,怎么算出弧度的长度

b=6.8 b/2=3.4 h=1.4 (r-h)²+(b/2)²=r²
(r-1.4)²+(3.4)²=r²
r²-2.8r+1.4²+3.4²=r²
-2.8r+1.96+11.56=0
2.8r=1.96+11.56=13.52
r=169/35
sin(A/2)=3.4/(169/35)=0.70414201183431952662721893491124
A/2=44.76° A=89.52
弧长=2πr*A/360=7.5442960439896077709679842637361（米）