AB*AC = 4 ------------------ (1)
min(AP^2) = AC^2 = 2 ------------------ (2)
max(AP^2) = AB^2 = 8------------------ (3)
所以A B 都不对,应该是AB*AC可能大于等于小于PA^2
PB + PC = 2 >= 2(PB*PC)^(1/2),当PB=PC=2/2=1时候“=”成立, 所以
PB*PC AB*AC > PB*PC
(2)(4) ----> PA^2 > PB*PC