Combining the two sentences by using a conjunct:

Question

The term papers were very brief. They were better than I had expected.
The term papers were very brief but they were better than I had expected，对吗？

Answer 1

完全正确，也可以把but替换成and.

Answer 2

这个题目是要求把两个句子用连词连接起来，用but没有问题，当然还可以在句子开头用Although，句子写成：Although the term papers were very brief, they were better than I expected.

Answer 3

题主给的句子并不太好。

如果是考试答卷，在我看来能得一半分就是好的，理由是but的使用不当：very brief不仅没有丝毫贬义，而且通常是含有褒义的（与short不同，brief常译成“简短”），所以跟后面具有褒义的分句连接时，应该用并列连词and而非but.

本题可以使用的连词不止一个：

1. The term papers were very brief and they were better than I had expected. （学期论文都很短小，而且都比我期待/想象的要好）

2. Although the term papers were very brief, they were better than I had expected.(虽然…，但是…)

3. Whereas the term papers were very brief, they were…expected.(尽管…，但是…)

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