(12){\displaystyle \left({\frac {1}{2}}\right)}
(r+x)2+(R−x)2−(R−r)2−2(r+x)(R−x)cos(α)=0{\displaystyle (r+x)^{2}+(R-x)^{2}-(R-r)^{2}-2(r+x)(R-x)cos(\alpha )=0} R(r(1−cos(α))−x(1+cos(α)))+x⋅(1+cos(α))(r+x)=0{\displaystyle R(r(1-cos(\alpha ))-x(1+cos(\alpha )))+x\cdot (1+cos(\alpha ))(r+x)=0} R(r⋅(1−cos(α)1+cos(α))−x)+x⋅(r+x)=0{\displaystyle R\left(r\cdot \left({\frac {1-cos(\alpha )}{1+cos(\alpha )}}\right)-x\right)+x\cdot (r+x)=0}
(1−cos(α){\displaystyle (1-cos(\alpha )}