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U3=a+(3-1)b
-10=a+2b
Un=a+(n-1)b
U5=a+(5-1)b
-4=a+4b
a+2b=-10
a+4b=-4
dikurangi
-2b=-6
b=3
a+2b=-10
a+2.3=-10
a+6=-10
a=-10-6
a=-16
Sn=n/2(2a+(n-1)b)
S15=15/2(2(-16)+(15-1)3)
S15=15/2(-32+(14)3)
S15=15/2(-32+42)
S15=15/2(10)
S15=75