Diketahui deret aritmatika suku ke-3 = 14 dan suku ke-7 = 26.Tentukan suku ke-20 dan jumblah 18 suku pertama!

Bab : Barder Aritmatika
Tinɡkat : Kelas XI-SMA
Solusi :
Eliminasi substitusi utk cari a dan b
U3=a+2b=14
U7=a+6b=26
---------------(−)
-4b=−12
b=3

a+2(3)=14
a=14-6=8

Un=a+(n-1).b
U20=8+(20-1).3
=8+19(3)
=8+57
=65

S18=18/2.(8+65)
=9.73
=657

u3 = 14
14 = a + (3-1)b
14 = a + 2b

u7 = 26
26 = a + (7-1)b
26 = a + 6b
Eliminasi
a+2b = 14
a+6b = 26 -
= -4b = -12
= b = -12/-4
b = 3
Subtitusi
a+2b = 14
a + 2.3 =14
a+ 6 =14
a = 14-6
a= 8
u20 = a + (n-1)b
u20 = 8 + (20-1)3
u20 = 8 + (19)3
u20= 8 + 57
u20 = 65
S18 = n/2 (2a+ (n-1)b)
S18 = 18/2 (2.8+(18-1)3
S18 = 9 (16+17.3)
S18 = 9 (16+ 51)
S18 = 9 (67)
S18 = 603