3．设矩阵A= <img src="https://zk.tikuw.cn/lqwvjeImg3/0/f/5a/0f5a7b9af6ead3e865325f7c8029dac1.jpg" title="3" alt="3" suffix="jpg"/> ，则二次型f(x<sub>1</sub>,x<sub>2</sub>,x<sub>3</sub>)=x<sup>T</sup>Ax的规范形为 A．z<sub>1</sub><sup>2</sup><sub></sub>- z<sub>2</sub><sup>2</sup><sub></sub>- z<sub>3</sub><sup>2</sup> B．z<sub>1</sub><sup>2</sup>+z<sub>2</sub><sup>2</sup>-z<sub>3</sub><sup>2</sup> C．z<sub>1</sub><sup>2</sup>+z<sub>2</sub><sup>2</sup> D．z<sub>1</sub><sup>2</sup>-z<sub>2</sub><sup>2</sup>