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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
4/y+2/7 = (4/7)*y // - (4/7)*y
4/y-((4/7)*y)+2/7 = 0
(-4/7)*y+4/y+2/7 = 0
4*y^-1-4/7*y^1+2/7*y^0 = 0
(2/7*y^1-4/7*y^2+4*y^0)/(y^1) = 0 // * y^2
y^1*(2/7*y^1-4/7*y^2+4*y^0) = 0
y^1
(-4/7)*y^2+(2/7)*y+4 = 0
(-4/7)*y^2+(2/7)*y+4 = 0
DELTA = (2/7)^2-(4*4*(-4/7))
DELTA = 452/49
DELTA > 0
y = ((452/49)^(1/2)-(2/7))/(2*(-4/7)) or y = (-(2/7)-(452/49)^(1/2))/(2*(-4/7))
y = -7/8*((452/49)^(1/2)-2/7) or y = 7/8*((452/49)^(1/2)+2/7)
y in { -7/8*((452/49)^(1/2)-2/7), 7/8*((452/49)^(1/2)+2/7)}
y in { -7/8*((452/49)^(1/2)-2/7), 7/8*((452/49)^(1/2)+2/7) }
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