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U6 = a+5b = 15
U11 = a+10b = 25 _
-5b = -10
b = 2
a+5(2) = 15
a = 15-10
a = 5
Sn = n/2 (2a+(n-1)b)
S16 = 16/2 (2.5+15.2)
= 8 (10+30)
= 8 (40)
= 320
semoga membantu :)
U11= 25
-> Un =a + (n-1)b
Eliminasi!
U6 = a + (6-1)b => a+ 5b=15
U11=a +(11-1)b=> a+10b=25 -
-5b =-10
b = 2
Subtitusi!
U6 => a +5b=15
a +5.2=15
a +10 = 15
a = 15-10 = 5
Jumlah 16 suku pertama =.....?
~ Diketahui : n = 16
a = 5
b = 2
Sn = n/2 [2a+(n-1)b]
S16= 16/2[2.5+(16-1)2]
= 8 [10+15.2]
= 8 [10+30]
= 8 . 40
= 320