Exercise 5 draft

Added Exercise 5 but incomplete

Informatik_I/Exercise_5/Task_3/main.md

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+Please write your solution here.
+For your convenience we have added a template for your answers below.
+
+Note: Remember to also describe the problem of the function.
+
+# Function `c++|is_even`:
+
+The problem with this function is that it does not return a bool if the number `i` is uneven.
+
+```c++
+// PRE:  n/a
+// POST: returns true if i is even. If not it returns false.
+bool is_even(int i) {
+  if (i % 2 == 0) return true;
+  else return false;
+}
+```
+
+# Function `c++|invert`:
+
+The problem with this function is
+
+```c++
+// PRE: x is positive or negative but not zero.
+// POST: returns inverse with respect to multiplication of x.
+double invert(int x) {
+  // TODO: Fix code below.
+  if (x != 0) {
+      return 1.0 / x;
+  }
+  else
+    std::cout << "0 has no inverse!";
+}
+```
+