Question

U3 = 7, U9 = 19. S19 deret tersebut adalah?
Mohon pembahasannya yaa

Answer 1

Un = a + (n-1)b
U3 = a + (3-1)b
U3 = a + 2b

U9 = a + (9-1)b
U9 = a + 8b

a + 8b = 19
a + 2b = 7_-
6b = 12
b = 12/6
b = 2

a + 2b = 7
a + 2(2) = 7
a + 4 = 7
a = 7 - 4
a = 3

Sn = 1/2 n (2a + (n-1)b)
S19 = 1/2 . 19 (2.3 + (19-1)2)
= 1/2 . 19 . (6 + (18)(2)
= 1/2 . 19 . (6 + 36)
= 1/2 . 19 . 42
= 21 . 19
= 399

Answer 2

U9=> a + 8b = 19
U3=> a + 2b = 7   _
               6b = 12
                 b = 2
subtitusikan
a + 2(2) = 7
a + 4 = 7
   a  = 7 -4
   a  = 3
jumlah suku
S19 = n/2(2a + (n-1)b )
       = 19/2 ( 2 x 3 + ( 19-1) 2)
       = 19/2 ( 6 + 36)
       = 19/2 x 42
       = 19 x 21
       = 399

jawabnnya 399