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U9 = 4 >> a + 8b = 4 ..... (2)
eliminasi pers. 1 dan 2 maka diperoleh
b = 3 , a = -20
U7 = a + 6b = -20 + (6x3) = -20 + 18 = -2
U9=4
-) Un a+(n-1).b
U4 = a+ (4-1).b
=a+3b= -11
-) U9 = a+9-1.b
=a+8b = 4
pake eliminasi
a+3b= -11
a+8b=4
_________ -
-5b=-15
b=3
subtitusi
a+3b=-11
a+3(3)=-11
a+9=-11
a=-11-9
a=-20
** U7 = a+(n-1).b
-20 + (7-1).3
-20+18
-2
2. Un = a + (n-1).b
U5 = a + 4b = 40
U12 = a + 11b = 117
eliminasi
a+4b=40
a+11b=117
__________ -
-7b = - 77
b= 11
subtitusi
a+4b=40
a+4(11)=40
a=40-44
a=-4
Sn = n/2(2a+(n-1).b)
S17 = 17/2 ( 2(-4)+(17-1).11
= 17/2(-8+176)
17/2(164)
17×82 = 1394