The epicentre is the intersection of the perpendicular bisector of the three sides of the triangle.

"the projection of point a on the plane sbc is the outer center of the triangle sbc",

The projection of point a in the plane bsc is d,

So: point D is the outer center of triangle sbc (that is, point D is the intersection of the perpendicular lines of BS, SC and bc).

∵? "D is at the midpoint of the hypotenuse BC of the triangle bsc" and AB=AC,

∴△ABC is an isosceles triangle, so AD is perpendicular to BC and sd = cd = bd.

AD=AD，

AS=AB，

DS=DB，

(SSS)，

So △ ADB △ ads,

So ∠ADB=∠ADS=? 90 ,

AD⊥BC,AD⊥DS，

BC and DS are on BSC plane,

BC and DS intersect at point d,

∴AD⊥ Bachelor of Science in Plane.

I don't know if it's right. Ha ha. . ? I drew another picture myself.