大神笔记： http://ai-start.com/ml2014/html/week4.html#header-n634
8.7 Suppose you have a multi-class classification problem with 10 classes. Your neural network has 3 layers, and the hidden layer (layer 2) has 5 units. Using the one-vs-all method described here, how many elements does Θ(2)
50 55 60√ 66
多类分类器包含十个类，第三层激活单元为10作θ行，第二层有5个单元，加上一个偏差，则为6个作θ列，θ维度应为10*6