# 三角学

## Calculating the side length of a triangle

Calculates the length of the hypotenuse of a right-angle triangle with an angle of 2 radians and opposite side length of 80.

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

let hypotenuse = side_length / angle.sin();

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


## Verifying tan is equal to sin divided by cos

Verifies tan(x) is equal to sin(x)/cos(x) for x = 6.

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

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

assert_eq!(a, b);
}


## Distance between two points on the Earth By default, Rust provides mathematical float methods such as trigonometric functions, square root, conversion functions between radians and degrees, and so forth.

The following example computes the distance in kilometers between two points on the Earth with the Haversine formula. Points are expressed as pairs of latitude and longitude in degrees. Then, to_radians converts them in radian. sin, cos, powi and sqrt compute the central angle. Finally, it’s possible to calculate the distance.

fn main() {
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 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
);
}