Bài 1. Cho tứ diện SABC có  ABC vuông cân tại B, SA  (ABC), cho SA = a 2 , AB = a. Gọi H, K lần lượt là chân đường vuông góc của A lên các cạnh SB, SC. 1/ Chứng minh : a) BC  (SAB) b) AH  SC c) (SAB)  (SBC). D) SC  (AHK) 2/ Tính góc giữa hai mặt phẳng: a, SB với (ABC) b, (SBC) và (ABC) 3/ Tìm khoảng cách từ A đến (SBC), khoảng cách từ B đến (SAC)

(co 3) the times that customers spend in a book store are normally distributed with a mean of 39.5 minutes and a standard deviation of 9.4 minutes. a random sample of 25 customers has a mean of 36.1 minutes or less. would this outcome be considered unusual, so that the store should reconsider its displays? no, the probability of this outcome at 0.035, would be considered usual, so there is no problem yes, the probability of this outcome at 0.035, would be considered unusual, so the display should be redone no the probability of this outcome at 0.359 would be considered usual, so there is no problem yes, the probability of this outcome at 0.965 would be considered unusual, so the display should be redone