动手动脑5

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因为IEEE 754标准规定：A NaN value is used to represent the result of certain invalid operations such as dividing zero by zero. 

for example, 1.0/0.0 has the value positive infinity, while the value of 1.0/-0.0 is negative infinity.

意思就是NaN=0.0/0.0  infinity=正浮点数/0.0  negative infinity=负浮点数/0.0  所以浮点数除以0.0不会报错

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