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a+2b + a+4b + a+6b = 15
3a + 12b = 15
U6 +U8 = -U4
a+5b + a+7b = -a-3b
3a + 15b = 0
3a + 15b = 0
3a + 12b = 15
__________ -
3b = -15
b = -5 , a = 25
suku ke 25 = U25= a + ( 25 -1 )b
= 25 + 24 × -5 = -95