简答题:已知二次型<img src="http://www.weimei168.com/uploads/202208/16126716783471.png" width="115" height="30" /><img src="http://www.weimei168.com/uploads/202208/16126716848271.png" width="235" height="32" />的秩为2． <br /> 求：(1)参数c； <br /> (2)利用正交变换将二次型 <img src="http://www.weimei168.com/uploads/202208/161267169191041.png" width="20" height="27" />化为标准形，并写出所用的正交变换对应的正交矩阵．