这个等式对于任何大于2的数值都不成立。
The equation is impossible for any value of n greater than 2.
对大多数值来说，这个循环都挺管用的，但一般来说用浮点数来测试等式是很危险的。
For most values of a this works fine, but in general it is dangerous to test float equality.
含有字母的恒等式对其中字母的任何一组数值都成立。
An identity involving letters is true for any set of numerical values of the letter in it.
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