如图所示,小磁针静止在地球赤道正上方,请标出此时小磁针的N、S极.
Her mother often forgets the door but this morning she remembered it.
(Ⅰ)证明: PC⊥PB ;
(Ⅱ)若点 E 在线段 PC 上,且 PC=3PE ,求三棱锥 A−EBC 的体积.