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a+(n-1)b = -11
a+(3-1)b = -11
a+2b = -11
Sn ke 20
230= n/2 (2a+(n-1)b)
230=20/2 (2a+(20-1)b)
23= 2a+19b
persamaan =
2a+4b=-22
2a+19b=23
diperoleh -15b=-45
jadi b = 3
dan a= -17
suku ke 10 adalan a+(n-1)b = -17 + (10-1)3 =20