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U₁₇ = 81
Un = a
+(n-1)b U₅ = a
+(5-1)b
21 = a
+4b ........... [1] U₁₇ = a
+(17-1)b
81 = a+16b
........... [2] eliminasi pers
[1] dan [2] :
a+4b = 21
a+16b = 81
_________--
-12b = -60
b =
60/12
b = 5 substitusi
nilai b ke
salah satu
persamaan
21 = a + 4b
21 = a + 4(5) 21 = a + 20
a = 21 -
20
a = 1 sehingga :
Un = a
+(n-1)b
U₂₅ =
1+(25-1)5
U₂₅ = 1 + 24 x 5
U₂₅ = 1 +
120
U₂₅ = 121
<-----
Jawaban suku ke-25 dengan
demikian :
Sn = n/2 {2a
+(n-1)b}
S₂₅ =
25/2 {2(1) + (25-1)5}
S₂₅ =
25/2 {2+(24)
5}
S₂₅ =
25/2 {2+120} S₂₅ = 25/2
(122)
S₂₅ = 1525
<-~-~
Jawaban
Jumlah 25 suku
pertama.