20+16+12+8+... tentukan suku ke 8 dan jumlah 8 suku

Dik:
a = 20
b = -4

U8 = a+ 7b
U8 = 20 + 7(-4)
U8 = 20 - 28
U8= -8

S8 = 8/2 (20-8)
S8 = 4 × 12
S8 = 48

Semoga membantu :)

a = U1 = 20 U2 = 16 Beda b = U2 - U1 = 16 - 20 = -4 U8 = a + 7b = 20 + 7(-4) = -8 S8= n/2 (a + U8) = 8/2 (20 + (-8) = 4 ( 12) = 48