大约一半的习题有答案，取二或三位有效数字。
Answers are provided to about half the problems, and are rounded off to two or three significant figures.
四十年前，光谱学的测量结果最多能保持六到七位有效数字。
Forty years ago, spectroscopy measurement results were given with six or seven digits at most.
通常，从右到左排列，最右边的数字是最低有效位。
Usually, you order them from right to left, with the right-most digit being the least significant digit.
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