圖像來源，BBC Chinese / Lok Lee
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，这一点在下载安装 谷歌浏览器 开启极速安全的 上网之旅。中也有详细论述
优点：输出在 (−1,1)，比 sigmoid 居中，对梯度更友好
Hard difficulty hints, answers for Feb. 27 PipsNumber (6): Everything in this space must add up to 6. The answer is 4-0, placed horizontally; 2-6, placed vertically.