diketahui barisan aritmatika: u3 =19 dan u6=37 tentukan: a. beda,a b. u20 c. s20

Dik :
- U3 = 19
- U6 = 37
Dit :
a. b, a
b. U20
c. S20
jawab.

A. b,a
U3=a + 2b = 19 | x5 | 5a + 10b=95
U6=a + 5b = 37 | x2 | 2a + 10b=74
------------- -
3a =21
a =21/3
a = 7

U3 = a + 2b = 19
= 7 + 2b = 19
= 2b = 19 - 7
= b = 12/2
b = 6

B. U20
U20 = a + (n-1)b
= 7 + (20-1)6
= 7 + 19 x 6
= 7 + 114
= 121
C. S20
Sn = n (2a + (n-1)b)
S20 = 20 (2 x 7 + (20-1)6)
S20 = 20 ( 14 + 19 )6)
S20 = 20 x 33 x 6
S20 = 3960