# Secant Theorem (Wu, 1987)   
Let(A=[x1,0], C=[-x1,0], O=[0,x2], B=[x3,x4], D=[x5,x6], S=[x7,0]):
Secant := Theorem(
    {equidistance(O,A,O,B), equidistance(O,A,O,D), collinear(B,D,S)},
    sdistance(S,A)*sdistance(S,C)-sdistance(S,B)*sdistance(S,D), [x4,x6,x7]);
