Jawabanmu

2014-02-23T23:03:48+07:00
 S_{20} = 440
 S_{18} = 360
 S_{20}-S_{18}=2b = 440-360
 2b = 120 
 b = .....
 S_{18} = 360 = a + 17b
 a = ...
empat suku pertama : a, a+b, a+2b, a+3b
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