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U3 + U4 = 45
a.r² + a.r³ = 45
ar ( r + r²) = 45
60 (r + r²) = 45
r² + r = 45/60
r² + r = 3/4
r² + r - 3/4 = 0
4r² + 4r - 3 = 0
(2r - 1)(2r + 3) = 0
2r - 1 = 0 atau 2r + 3 = 0
2r = 1 atau 2r = -3 (tak dikehendaki)
r = 1/2 (r positif yang diminta)
U₂ = 60
U₂ = a.r = 60
ar² + ar³ = 45
ar (r²+r) = 45
60 (r²+r) = 45
60r² + 60r = 45
60r² + 60r - 45 = 0
(60r²+90)-(30+45)
30(2r+3) - 15(2r+3)
(30r-15)(2r+3)
rasio positif
30r - 15 = 0
r = 15/30
r = 1/2