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s − 120 = 1000
t +1
2

(s − 120)t + 1  = 1000
2

(s − 120) 1000 + 1  = 1000
s 2

(s − 120) 2000 + s  = 1000
2s

(s − 120)(2000 + s) = 2000s

2000s + s2 − 240000 − 120s = 2000s

s2 − 120s − 240000 = 0

s = 120 ± (120)2 − 4(1)(−240000)
2

s = 120 ± 974400
2

s = 120 ± 987.1 .s = 120 + 987.1 = 553.6
2
2
. 60
x ()

.3

.5