tolong di jawab yaaaaaa

a = 20
b = 3

a) Un = a + (n-1)b
          = 20 + (n-1)3
          = 20 + 3n - 3
          = 3n + 17

b) Un = 3n + 17
    U12 = 3 (12) + 17
            = 36 + 17
            = 53 kursi

c. Sn = n/2 ( 2a + (n-1)b)
    S15 = 15 / 2 ( 2 x 20 + (15 - 1)3)
            = 15 / 2 (40 + 42)
            = 15 / 2 x 82
            = 615 kursi