# 三角学

## 计算三角形的边长

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

let hypotenuse = side_length / angle.sin();

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


## 验证正切（tan）等于正弦（sin）除以余弦（cos）

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

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

assert_eq!(a, b);
}


## 地球上两点之间的距离

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
);
}