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x in (-oo:+oo)
200/400 = x/1680 // - x/1680
200/400-(x/1680) = 0
1/2-1/1680*x = 0 // - 1/2
-1/1680*x = -1/2 // : -1/1680
x = -1/2/(-1/1680)
x = 840
x = 840
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