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D( y )
y = 0
y+400 = 0
y = 0
y = 0
y+400 = 0
y+400 = 0
y+400 = 0 // - 400
y = -400
y in (-oo:-400) U (-400:0) U (0:+oo)
0.1 = 2/(y+400)-(8/y) // - 2/(y+400)-(8/y)
8/y-(2/(y+400))+0.1 = 0
8/y-2*(y+400)^-1+0.1 = 0
8/y-2/(y+400)+0.1 = 0
(-2*y)/(y*(y+400))+(8*(y+400))/(y*(y+400))+(0.1*y*(y+400))/(y*(y+400)) = 0
8*(y+400)+0.1*y*(y+400)-2*y = 0
0.1*y^2+6*y+40*y+3200 = 0
0.1*y^2+46*y+3200 = 0
0.1*y^2+46*y+3200 = 0
0.1*y^2+46*y+3200 = 0
DELTA = 46^2-(0.1*4*3200)
DELTA = 836
DELTA > 0
y = (836^(1/2)-46)/(0.1*2) or y = (-836^(1/2)-46)/(0.1*2)
y = 5*(2*209^(1/2)-46) or y = 5*(-2*209^(1/2)-46)
(y-5*(-2*209^(1/2)-46))*(y-5*(2*209^(1/2)-46)) = 0
((y-5*(-2*209^(1/2)-46))*(y-5*(2*209^(1/2)-46)))/(y*(y+400)) = 0
((y-5*(-2*209^(1/2)-46))*(y-5*(2*209^(1/2)-46)))/(y*(y+400)) = 0 // * y*(y+400)
(y-5*(-2*209^(1/2)-46))*(y-5*(2*209^(1/2)-46)) = 0
( y-5*(-2*209^(1/2)-46) )
y-5*(-2*209^(1/2)-46) = 0 // + -5*(-2*209^(1/2)-46)
y = -(-5*(-2*209^(1/2)-46))
y = 5*(-2*209^(1/2)-46)
( y-5*(2*209^(1/2)-46) )
y-5*(2*209^(1/2)-46) = 0 // + -5*(2*209^(1/2)-46)
y = -(-5*(2*209^(1/2)-46))
y = 5*(2*209^(1/2)-46)
y in { 5*(-2*209^(1/2)-46), 5*(2*209^(1/2)-46) }
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