Added unimplemented methods in task5 and ran the tests

hw1/task4/Cargo.toml

+[package]
+name = "task4"
+version = "0.1.0"
+authors = ["Lorenz Bung <lorenz.bung@googlemail.com>"]
+
+[dependencies]

hw1/task4/src/lib.rs

 pub fn square_of_sum(n: i32) -> i32 {
-    unimplemented!();
+    let mut i: i32 = 0;
+    for x in 1..n+1 {
+        i += x;
+    }
+    i.pow(2)
 }
 
 pub fn sum_of_squares(n: i32) -> i32 {
-    unimplemented!();
+    let mut i: i32 = 0;
+    for x in 1..n+1 {
+       i += x.pow(2); 
+    }
+    i
 }
 
 pub fn difference(n: i32) -> i32 {
-    unimplemented!();
+    square_of_sum(n) - sum_of_squares(n)
 }