Major chemical study of the concept, usually severallearning;
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usually several learning
通常是几门学问
以上为机器翻译结果，长、整句建议使用 人工翻译 。
The major disadvantages of the current learning methods are: (1) one-type cost assumption is not agreed with the actual case where there usually exist several types of costs;
这些学习方法存在的主要缺点有：（1）单一代价假设不符合现实存在的多种代价的实际情况；
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