修订chapter9

docs/chapter9/chapter9.md

@@ -148,8 +148,8 @@ $$
 $$
 \frac{\partial}{\partial x}\left(A^{-1}\right)=-A^{-1}\frac{\partial A}{\partial x}A^{-1}
 $$
-参考资料
-Petersen, K. B., & Pedersen, M. S. *The Matrix Cookbook*.
+参考资料<br>
+Petersen, K. B., & Pedersen, M. S. *The Matrix Cookbook*.<br>
 Bishop, C. M. (2006). *Pattern recognition and machine learning*. springer.