suku ke 6 dan suku ke 12 barisan aritmatika adalah 35 dan 65 tentukan jumlah ke 30 suku pertama

U6 = 35
U12 = 65
S30 = ... ?
U6 = a + 5b = 35
U12 = a + 11b = 65
a = 35 - 5b
U12 = 35 - 5b +11b = 65
                           6b = 30
                             b = 5
U12 = a + 11(5) = 65
                        a = 65 - 55
                        a = 10

Sn = n/2 (2a + (n-1)b)
S30 = 30/2 (2(10) + (29)5)
       = 2475