Suku pertama Geometri= 5 Dan jumlah 3 suku pertama = 65 Tentukan sudut ke 4nya ....

jawab

a = 5
S3 = 65
a (r³ -1)/(r - 1) = 65
(r³ -1)/(r -1) = 65/5  =13
(r-1)(r² + r + 1) /(r -1) = 13

r² + r + 1 = 13
r²+ r - 12 = 0
(r +4)(r -3) =0
r = - 4 atau r = 3

Suku ke 4 = U4 =  a r³
a= 5, r = - 4 → u4 = 5 (-4)³ = - 320
a = 5, r = 3  → u4 = 5 (3)³ = 135