Tweak beta computation in the Gamma distribution

Author: newpavlov

rand_distr/src/gamma.rs

@@ -620,8 +620,12 @@ where
         if a > F::one() {
             // Algorithm BB
             let alpha = a + b;
-            let beta =
-                ((alpha - F::from(2.).unwrap()) / (F::from(2.).unwrap() * a * b - alpha)).sqrt();
+
+            let two = F::from(2.).unwrap();
+            let beta_numer = alpha - two;
+            let beta_denom = two * a * b - alpha;
+            let beta = (beta_numer / beta_denom).sqrt();
+
             let gamma = a + F::one() / beta;
 
             Ok(Beta {