suatu barisan aritmetika diketahui U2 + U5 = 1 dan U4 + U7 = 21. a.suku ke-15 b.jumlah 12 suku pertama

Dik. barisan aritmatika : U2+U5 = 1 & U4 + U7 = 21
Dit. a)U15 & b) S12? 

Jawab...

U2 = a + b 
U5 = a + 4b 
     ------------------ (+)
@ 2a + 5b = 1 (Pers. 1)

U4 = a + 3b 
U7 = a + 6b 
     ------------------- (+)
@ 2a + 9b = 21 (Pers. 2)

Eliminasi Pers. 1 & Pers. 2 :

2a + 5b = 1
2a + 9b = 21
-------------------- (-)
@ -4b = -20
@ b = 5

a :
2a + 5b = 1
2a + 5(5) = 1
2a + 25 = 1
2a = 1 - 25
2a = -24
a = -12

#a 
U5 = a + 4b
U5 = -12 + 4(5)
U5 = -12 + 20
U5 = 8

#b 
Sn = n/2 (2a + (n-1) b)
S12 = 12/2 (2(-12) + (11) (5))
S12 = 6 ( -24 + 55)
S12 = 6 (31)
S12 = 186

Semoga membantu... ^_^