Let the sum of n, 2n, 3n terms of an A.P., be S₁, S₂, S₃, respectively, show that

Let a be the first term and d be the common difference.

∴                                               ...(i)
                                             ...(ii)
                                             ... (iii)
Subtracting (i) from (ii), we get
        
                    
    [B using (3)]
Hence,