diketahui deret aritmatika U7 =16 dan U13= 97 maka S16

Rumus suku ke-n        = Un = a + (n-1) b
Rumus jumlah suku n = Sn = n/2 (2a + (n-1) b)
U7   = 16  => a + 6 b   = 16
U13 = 97  => a + 12 b = 97
                    ____________-
                           -6 b   = -81
                              b    = 13,5
=> a + 6 b        = 16
=> a + 6 (13,5) = 16
=> a + 81         = 16
=> a                 = -65

S16 = 16/2 (2 x (-65) + 15 x 13,5)
S16 = 8 ( -130 + 202,5)
S16 = 8 (72,5)
S16 = 580