Tentukan suku jumalah 8 suku pertama -2,-4,-6

diket : a = -2
           b = -2
ditanya : jumlah suku ke 8
jawab : Sn = [tex] \frac{n}{2} [/tex] (2a + (n-1)b)
                  = [tex] \frac{8}{2} [/tex] (2.-2 + (8-1)-2)
                  = 4 (-4 + (7)-2)
                  = 4 (-4 + (-14))
                  = 4 (-18)
                  = -72