Mathematically, θ is usually pronounced "Sita" or "apostrophe" and is the eighth letter of the Greek alphabet. In trigonometric functions, θ usually represents angle, such as sine function sin(θ) and cosine function cos(θ). In polar coordinate system, θ usually represents the polar angle, indicating the included angle between a point and the polar axis. In addition, in probability theory and statistics, θ usually represents parameters, such as the mean μ and variance σ of normal distribution. Can be expressed as N(μ, σ? ), where μ and σ? It's theta. In a word, θ is widely used in mathematics, representing different concepts and meanings.