Prop. XXIII. Prob. XV
Trajectoriam describere quæ per data quatuor puncta transibit, & rectam continget positione datam.
Cas. 1. Dentur tangens HB, punctum contactus B, & alia tria puncta C, D, P. Junge BC, & agendo PS parallelam BH, & PQ parallelam BC, comple parallelogrammum BSPQ. Age BD secantem SP in T, & CD secantem PQ in R. Deniq; agendo quamvis tr ipsi TR parallelam, de PQ, PS abscinde Pr, Pt ipsis PR, PT proportionales respective; & actarum Cr, Bt concursus d (per Corol. 2. Lem. XX) incidet semper in Trajectoriam describendam.