theme changed

labs/lab8/Progrssion/ArithmeticProgression.cs

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+using System;
+
+class ArithmeticProgression : IProgression
+{
+    private int a0;
+    private int b;
+
+    public ArithmeticProgression(int a0, int b)
+    {
+        this.a0 = a0;
+        this.b = b;
+    }
+
+    public int GetElement(int k)
+    {
+        return a0 + (k - 1) * b;
+    }
+
+    public int Sum(int n)
+    {
+        return (2 * a0 + (n - 1) * b) * n / 2;
+    }
+}