SetAccuracy
 Usage
• SetAccuracy[expr, n] 把expr中所有数设置为具有n个数位的准确度并返回expr.
Notes
• 当使用SetAccuracy来提高一个数的准确度时,该数将被用0填充.0采用的基底为2.在基底为10时,附加数位通常不是0. • 即便所得到的重要数位的数比$MachinePrecision小,SetAccuracy仍返回一个任意精度的数. • 当 expr 包含机器精度的数时, 在不同计算机系统上SetAccuracy[expr, n]能给出不同的结果. • SetAccuracy首先打开一个数的内部二进制表示中所有隐藏的特殊数位,且只有用完这些之后才添加尾数0. • 在任何计算机系统上0.004``25生成一个所有末尾数字为0，准确度为25的数. • SetAccuracy[expr, n] 不修改 expr 本身.
 Further Examples
When an expression contains machine-precision real numbers, Mathematica tries to do all computations at machine-precision level.
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This causes a loss of accuracy. One of the inputs had an accuracy of 30 digits but the result has an accuracy of only 16 digits.
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By raising the accuracy of each subexpression, you can increase the accuracy of the overall result.
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