Frequency, that is, the number of times to complete periodic changes in time, is a quantity that describes the frequency of periodic motion. It is usually represented by the symbol F or V, with the unit of one second and the symbol s- 1. The frequency of alternating current refers to the number of times it changes periodically in unit time. The unit is hertz (Hz), which is the reciprocal of the period. The frequency of alternating current in daily life is generally 50Hz or 60Hz, while the frequency of alternating current involved in radio technology is generally large, reaching the measurement of kilohertz (KHz) or even megahertz (MHz).

Nature:

1. When the number of repeated experiments n increases gradually, the frequency fn(A) is stable and gradually stabilizes at a certain constant, which is the probability of event A. This "frequency stability" is also called statistical regularity.

2. Frequency:

(1) Nonnegativity: 0 is less than or equal to fn(A) is less than or equal to 1.

(2) Normality: fn (ω) = 1 (Note: ω stands for sample space).

(3) additivity.

3. Frequency is not equal to probability. According to Bernoulli's law of large numbers, when n tends to infinity, the frequency fn(A) approaches the probability P(A) in a certain sense.