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D( x )
30*x+10.5 = 0
30*x+10.5 = 0
30*x+10.5 = 0
30*x+10.5 = 0 // - 10.5
30*x = -10.5 // : 30
x = -10.5/30
x = -0.35
x in (-oo:-0.35) U (-0.35:+oo)
900/(30*x+10.5) = 40 // - 40
900/(30*x+10.5)-40 = 0
900/(30*x+10.5)+(-40*(30*x+10.5))/(30*x+10.5) = 0
900-40*(30*x+10.5) = 0
480-1200*x = 0
(480-1200*x)/(30*x+10.5) = 0
(480-1200*x)/(30*x+10.5) = 0 // * 30*x+10.5
480-1200*x = 0
480-1200*x = 0 // - 480
-1200*x = -480 // : -1200
x = -480/(-1200)
x = 2/5
x = 2/5
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