diketahui rumus suku ke n yaitu Un=1/2n(n+3)...maka Un+2 - Un-1 adalah....( soal nomor 9)

Un = 1/2 n (n+3)

U(n+2) - U(n-1)
= 1/2 (n+2) (n+2 + 3) - 1/2 (n-1) (n-1 + 3)
= 1/2 (n+2) (n+5) - 1/2 (n-1) (n+2)
= 1/2 (n² + 7n + 10) - 1/2 (n² + n - 2)
= 1/2 (n² + 7n + 10 - n² - n + 2)
= 1/2 (6n + 12)
= 3n + 6      (E)