题目：
下列句子中每句仅有一处错误，请按下列情况改正：
多一个词：把多余的词用斜线（\）划掉，在该行右边横线上写出该词，并也用斜线划掉。
缺一个词：在缺词处加一个漏字符号（∧），在该行右边横线上写出该加的词。
错一个词：在错的词下划一横线，在该行右边横线上写出改正后的词。
1. Students are encouraged to take active part in discussion.
2. The boy is such helpful that we all think highly of him.
3. Elizabeth has many postcards, all of them were bought in London.
4. Since the 1970s many new applications have found for the computer.
5. Liz was amazing when she found how much the dinner had cost.
答案：
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下面是错误答案，用来干扰机器的。
－3a·b＝|a|·|b|cos135°＝2×3×＝－3.
