dari barisan geometri dik suku ke 5 adalah 48 dsn suku ke 7 adalah192 tentukan jumlah suku pertama

Bab Baris dan Deret
Matematika SMP Kelas IX


U5 = 48
a.r^4 = 48.

U7 = 192
a.r^6 = 192.

a.r^4 = 48
a.r^6 = 192. :
r^-2 = 1/4
1/r^2 = 1/4
r^2 = 4
r = 2.

Ditanya = a ?

a.r^4 = 48
a.2^4 = 48
16a = 48
a = 48/16
a = 3.

Jangan lupa memilih jawaban terbaik
Semoga membantu;)

U5 = 48
U7 = 192

U5 = a + 4b = 48
U7 = a + 6b = 192

substitusi :
=> a + 4b = 48 -> a = 48 - 4b
=> a = 48 - 4b -> a + 6b = 192
=> 48 - 4b + 6b = 192
=> -4b + 6b = 192 - 48
=> 2b = 144
=> b = 144/2
=> b = 72

=> a = 48 - 4b
=> a = 48 - (4×72)
=> a = 48 - 288
=> a = -240