R=85/2=42.5 cmr=51/2=25.5cmR-r=42.5-25.5=17cmL-l=√(30^2+17^2)=34.48cmL:R=(L-l):(R-r), L:42.5=34.48:17 ⇒ L=86.20cml=86.20-34.48=51.72cmLθ=85π, 86.20θ=85×3.1416 ⇒ 展开角θ=3.098rad=177.49°
