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x in (-oo:+oo)
((10000/3)*x+40000)*(21-x)+3*((10000/3)*x+40000) = 0
(10000/3*x+40000)*(21-x)+3*(10000/3*x+40000) = 0
(10000/3*x+40000)*(21-x)+3*(10000/3*x+40000) = 0
40000*x-10000/3*x^2+960000 = 0
40000*x-10000/3*x^2+960000 = 0
10000*(4*x-1/3*x^2+96) = 0
4*x-1/3*x^2+96 = 0
DELTA = 4^2-(-1/3*4*96)
DELTA = 144
DELTA > 0
x = (144^(1/2)-4)/(-1/3*2) or x = (-144^(1/2)-4)/(-1/3*2)
x = -12 or x = 24
10000*(x+12)*(x-24) = 0
10000*(x+12)*(x-24) = 0
( x+12 )
x+12 = 0 // - 12
x = -12
( x-24 )
x-24 = 0 // + 24
x = 24
x in { -12, 24 }
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