本文根据晶体管非线性元件的特点，利用数学推导方法，从基础上分析了变频工作啸叫的原因，并提出了消除方法。
Based on the speciality of transistors non-linearity, using maths method, this article analyses the cause of whistle of frequency conversion circuit and provides method to eliminate.
利用数学方法，推导不对称公差标注与对称公差标注间的转换公式。
The transfer formula between asymmetric tolerance marking and symmetric tolerance marking is deduced by the mathematical method.
这个方法是根据周期函数的傅里叶级数的性质和利用数学的抽象性质，直接推导出另外三种形式的傅里叶变换。
According the basic properties of Fourier series of period function and using of the abstract nature of mathematics, the method directly derives other 3 kinds of Fourier transform.
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