# 三角函数

### 三角形边长计算

计算角为 2 弧度、对边长度为 80 的直角三角形的斜边长度。

    fn main() {
        let angle: f64 = 2.0;
        let side_length = 80.0;

        let hypotenuse = side_length / angle.sin();

        println!("Hypotenuse: {}", hypotenuse);
    }

### 验证 tan = sin / cos

    fn main() {
        let x: f64 = 6.0;

        let a = x.tan();
        let b = x.sin() / x.cos();

        assert_eq!(a, b);
    }

### 地球上两点间的距离

下面的代码使用 Haversine 公式 计算地球上两点之间的公里数。

    fn main() {
        let earth_radius_kilometer = 6371.0_f64;
        let (paris_latitude_degrees, paris_longitude_degrees) = (48.85341_f64, -2.34880_f64);
        let (london_latitude_degrees, london_longitude_degrees) = (51.50853_f64, -0.12574_f64);

        let paris_latitude = paris_latitude_degrees.to_radians();
        let london_latitude = london_latitude_degrees.to_radians();

        let delta_latitude = (paris_latitude_degrees - london_latitude_degrees).to_radians();
        let delta_longitude = (paris_longitude_degrees - london_longitude_degrees).to_radians();

        let central_angle_inner = (delta_latitude / 2.0).sin().powi(2)
            + paris_latitude.cos() * london_latitude.cos() * (delta_longitude / 2.0).sin().powi(2);
        let central_angle = 2.0 * central_angle_inner.sqrt().asin();

        let distance = earth_radius_kilometer * central_angle;

        println!(
            "Distance between Paris and London on the surface of Earth is {:.1} kilometers",
            distance
        );
    }