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a1 + a1r = a1(1 + r) = 90
a1r 2 = 24
1 + r = 90 = 15
r 2 24 4
15r 2 − 4r − 4 = 0
r = 4 ± 16 − 4 × (−4) ×15 = 4 ± 256 = 4 ± 16
2 × 15 30 30
.r2 = 4 − 16 = −2 r1 = 4 + 16 = 2
30 5 30 3
r1 = 2
3
. a1 4 = 24 ⇒ a1 = 9 × 24 = 54
9 4
r2 = −2
5
. a1 4 = 24 ⇒ a1 = 24 × 25 = 150
25 4