题目

When Duke University refused to accept Siobhan O’Dell, 17, by sending her a rejection letter, she refused to take it quietly. The teenager, from North Carolina, had applied to the famous college in her home state, but in March she was sent a letter telling her she had failed. Mirroring the standard format of college rejection letters, she wrote back：“Dear Duke University, thank you for the rejection letter of March 26, 2015. After careful consideration, I regret to inform you that I am unable to accept your refusal to offer me admission into the fall 2015 freshman class at Duke.已知正三棱锥，点都在半径为的球面上，若两两互相垂直，则球心到截面的距离为________。