1, as shown on the right, take the AC midpoint o.

Because VA equals VC

So VO is perpendicular to AC

Similarly, BO communicates vertically.

Because VO passes through BO and equals O.

So VB belongs to plane VOB.

So AC vertical VB

2. The midpoint of (1) ab;

(2)O is the outer center of triangle ABC;

(3)O is the center of triangle ABC;

Proof: (1)(2)

Connect OA' ob' oc

Because PA equals PB equals PC, and PO is the common side.

So right triangle AOP congruent right triangle BOP congruent right triangle COP

So OA equals OB equals OC.

So o is the center of triangle ABC.

So (1)(2) proves;

It is proved that (3) AO' CO is connected, and BC' AB is extended to D 'E.

Because PA is perpendicular to PC and Pb is perpendicular to PC.

So the PC vertical plane PAB

So PC vertical AB

Because PO is perpendicular to a.

So PO is perpendicular to ab, and po passing through PC is p.

So AB vertical company

Similarly BC vertical AO

So o is very concerned.

So (3) prove that

3. Not necessarily parallel.

Straight lines from two different planes can make the same angle as a plane, but they are not parallel.