已知
，二次型 
的秩为 2， (I) 求实数 a的值； (II) 求正交变换 x=Qy 将f 化为标准形 .
根据我国宪法，公民人身自由包括
MytrainarriveinNewYorkateighto'clocktonight.TheplaneIwouldliketotakefromthere______bythen.
Manypeopleattendvariouspubliclectures,chiefly______themselvesfamiliarwiththelatestdevelopmentofdifferentfields.
IheardthatyoureallyhadawonderfultimeatJohn'sbirthdayparty,_____?
