半径 R の円弧上の始点で幅 w1、終点で幅 w2 の拡幅円弧の長さの計算

L = R θ {\displaystyle L=R\theta }

k = w 2 − w 1 L {\displaystyle k={\frac {w_{2}-w_{1}}{L}}}

とすると、 d L = ( R + w 1 + k R θ ) d θ {\displaystyle dL=(R+w_{1}+kR\theta )d\theta } L w = ( R + w 1 ) θ + 1 2 k R θ 2 = L { 1 + w 1 R + k L 2 R } = L { 1 + 1 R ( w 1 + 1 2 k L ) } = L { 1 + 1 R ( w 1 + 1 2 ( w 2 − w 1 ) ) } = L { 1 + 1 R w 1 + w 2 2 } = ( R + w 1 + w 2 2 ) θ {\displaystyle {\begin{array}{rcl}Lw&=&\displaystyle (R+w_{1})\theta +{\frac {1}{2}}kR\theta ^{2}\\&=&\displaystyle L\left\{1+{\frac {w_{1}}{R}}+{\frac {kL}{2R}}\right\}\\&=&\displaystyle L\left\{1+{\frac {1}{R}}(w_{1}+{\frac {1}{2}}kL)\right\}\\&=&\displaystyle L\left\{1+{\frac {1}{R}}\left(w_{1}+{\frac {1}{2}}(w_{2}-w_{1})\right)\right\}\\&=&\displaystyle L\left\{1+{\frac {1}{R}}{\frac {w_{1}+w_{2}}{2}}\right\}\\&=&\displaystyle \left(R+{\frac {w_{1}+w_{2}}{2}}\right)\theta \end{array}}}