mohon bantuanya man teman

(2k + 25), (9 - k), (3k + 7), ...
b = U2 - U1 = U3 - U2
=> U2 + U2 = U3 + U1
=> (9 - k) + (9 - k) = (3k + 7) + (2k + 25)
=> 18 - 2k = 5k + 32
=> -2k - 5k = 32 - 18
=> -7k = 14
=> k = -2
(2k + 25), (9 - k), (3k + 7), ...
= (2(-2) + 25), (9 - (-2)), (3(-2) + 7), ....
= 21, 11, 1, ....
a = 21, b = 11 - 21 = -10
Sn = n/2 (2a + (n - 1)b)
= n/2 (2(21) + (n - 1)(-10))
= n/2 (42 - 10n + 10)
= n/2 (-10n + 52)
= -5n^2 + 26n

^ = pangkat