suatu barisan geometri U1+U3=10 dan U2+U4=5. Nilai dari U1+U4 adalah

U1 + u3 = a + ar² = 10
a(1+r²) = 10
1+r² = 10/a
U2 + u4 = ar + ar³ = 5
ar(1 + r²) = 5
ar ( 10/a) = 5
10r = 5
r = 5/10
r = ½

1 + (½)² = 10/a
1 + ¼ = 10/a
5/4 = 10/a
a = 4x10/5 = 8

U1 + U4 = a + ar³ = a(1 + r³)
= 8 x ( 1 + (½)³)
= 8 x ( 8/8 + ⅛)
= 8 x 9/8
= 9