Diketahui susunan deret 99,92,85,78,… tentukan S5 dsn J12

Deret Aritmatika

a=99
b=92-99=-7

Un=a+(n-1)b
=99+(n-1)-7
=99+(-7n+7)
=-7n+106

U5=-7(5)+106
=-35+106
=71
U12=-7(12)+106
=-84+106
=22

S12=n/2(a+U12)
=12/2(99+22)
=6(121)
=726
——
99, 92, 85, 78, 71, 64, 57, 50, 43, 36, 29, 22
total=726 (terbukti)

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