Hitunglah jumlah deret arimatika 15+12+9+..... sampai 15 suku !

Matematika

a = 15
b = 12 - 15 = -3

S15 = 15/2 (2a + 14b)
S15 = 15/2 (2.15 + 14.(-3))
S15 = 15/2 (30 - 42)
S15 = 15/2 (-12)
S15 = 15 × (-6)
S15 = -90

a=u1=15
b=u2-u1=12-15=-3
.
a=u1=15
b=u2-u1=12-15=-3
.
sn=n/2(2.a+(n-1)b
sn=15/2(2.15+(15-1)-3
=15/2(30+14X(-3))
=15/2 (30+(-42)
=15/2 X (-12)
=15 X (-12) : 2
= -90