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dit : Sn?
jawab :
u4 = u1+ 3b
41 = u1 + 3b
u9 = u1 + 8b
26 = u1 + 8b
u1 kita sebut variabel a
41 = a + 3b
26 = a + 8b
____________ - (subsitusi dikurangkan)
15 = -5b
b = 15/-5
b= -3
beda nya -3
26 = a+8b
26= a+ 8(-3)
26=a-24
26+24 = a
50 = U1
jadi, u1 nya 50
Sn = n/2 (a+(n-1)b)
S10 = 10/2 (50+(10-1)-3
= 5(50+9(3))
=5 (50+27)
=5(77)
= 385 (coba kali lagi)
jadi jumlah sepuluh suku pertama adalah 385
U9 = a+8b = 26
a+8b = 26
a+3b = 41
------------------ -
5b = -15
b = -15/5
b = -3
a+3b = 41
a+3(-3)=41
a-9=41
a=41+9
a=50
S10 = 10/2 (2a+(10-1)b)
= 5 (2(50)+9(-3))
= 5(100-27)
= 5(73)
= 365
Maaf, kalau salah.