In tetrahedral p-abc, the triangle ABC is an equilateral triangle, and PA = Pb = PC
It is obvious that BC is perpendicular to ad, BC is perpendicular to PD; because ad intersects PD, BC is perpendicular to pad, so BC is perpendicular to pa!
It is obvious that BC is perpendicular to ad, BC is perpendicular to PD; because ad intersects PD, BC is perpendicular to pad, so BC is perpendicular to pa!