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t in (-oo:+oo)
15 = 21.3*t-9.8*(1/2)*t^2 // - 21.3*t-9.8*(1/2)*t^2
15-(-9.8*(1/2)*t^2)-(21.3*t) = 0
4.9*t^2-21.3*t+15 = 0
4.9*t^2-21.3*t+15 = 0
DELTA = (-21.3)^2-(4*4.9*15)
DELTA = 21.3^2-294
DELTA = 159.69
DELTA > 0
t = ((21.3^2-294)^(1/2)+21.3)/(2*4.9) or t = (21.3-(21.3^2-294)^(1/2))/(2*4.9)
t = 0.10204082*((21.3^2-294)^(1/2)+21.3) or t = 0.10204082*(21.3-(21.3^2-294)^(1/2))
t in { 0.10204082*(21.3-(21.3^2-294)^(1/2)), 0.10204082*((21.3^2-294)^(1/2)+21.3) }
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