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d in (-oo:+oo)
4.85 = (1000/3)*d^2+3900*d+100 // - (1000/3)*d^2+3900*d+100
4.85-((1000/3)*d^2)-(3900*d)-100 = 0
(-1000/3)*d^2-3900*d-100+4.85 = 0
-1000/3*d^2-3900*d-95.15 = 0
DELTA = (-3900)^2-(-1000/3*(-95.15)*4)
DELTA = 15083133.333334602
DELTA > 0
d = (15083133.333334602^(1/2)+3900)/(-1000/3*2) or d = (3900-15083133.333334602^(1/2))/(-1000/3*2)
d = -3/2000*(15083133.333334602^(1/2)+3900) or d = -3/2000*(3900-15083133.333334602^(1/2))
d in { -3/2000*(15083133.333334602^(1/2)+3900), -3/2000*(3900-15083133.333334602^(1/2)) }
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