Sn=5n pangkat 2-7n ; u3-u5+u7

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

Sn = 5n² - 7n

Un = Sn - S(n - 1)

U3 = S3 - S2
U3 = (5 . 3² - 7 . 3 ) - (5 . 2² - 7 . 2)
U3 = (45 - 21) - (20 - 14)
U3 = 18

U5 = S5 - S4
U5 = (5 . 5² - 7 . 5) - (5 . 4² - 7 . 4)
U5 = (125 - 35) - (80 - 28)
U5 = 38

U7 = S7 - S6
U7 = (5 . 7² - 7 . 7) - (5 . 6² - 7 . 6)
U7 = (245 - 49) - (180 - 42)
U7 = 58

U3 - U6 + U7
= 18 - 38 + 58
= 38