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x in (-oo:+oo)
50*(x/90)*(90-x) = 10*x // - 10*x
50*(x/90)*(90-x)-(10*x) = 0
50*(x/90)*(90-x)-10*x = 0
5/9*x*(90-x)-10*x = 0
5/9*x*(90-x)-10*x = 0
40*x-5/9*x^2 = 0
40*x-5/9*x^2 = 0
5*x*(8-1/9*x) = 0
8-1/9*x = 0 // - 8
-1/9*x = -8 // : -1/9
x = -8/(-1/9)
x = 72
5*x*(x-72) = 0
5*x*(x-72) = 0
( 5*x )
5*x = 0 // : 5
x = 0
( x-72 )
x-72 = 0 // + 72
x = 72
x in { 0, 72 }
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