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D( y )
4*y-20 = 0
y-5 = 0
4*y-20 = 0
4*y-20 = 0
4*y-20 = 0 // + 20
4*y = 20 // : 4
y = 20/4
y = 5
y-5 = 0
y-5 = 0
y-5 = 0 // + 5
y = 5
y in (-oo:5) U (5:+oo)
1-7/(4*y-20) = -1/(y-5) // + -1/(y-5)
1-7/(4*y-20)-(-1/(y-5)) = 0
(y-5)^-1-7/(4*y-20)+1 = 0
1/(y-5)-7/(4*y-20)+1 = 0
(-7*(y-5))/((4*y-20)*(y-5))+(4*y-20)/((4*y-20)*(y-5))+(1*(4*y-20)*(y-5))/((4*y-20)*(y-5)) = 0
1*(4*y-20)*(y-5)-7*(y-5)+4*y-20 = 0
4*y^2-3*y-40*y+15+100 = 0
4*y^2-43*y+115 = 0
4*y^2-43*y+115 = 0
4*y^2-43*y+115 = 0
DELTA = (-43)^2-(4*4*115)
DELTA = 9
DELTA > 0
y = (9^(1/2)+43)/(2*4) or y = (43-9^(1/2))/(2*4)
y = 23/4 or y = 5
(y-5)*(y-23/4) = 0
((y-5)*(y-23/4))/((4*y-20)*(y-5)) = 0
((y-5)*(y-23/4))/((4*y-20)*(y-5)) = 0 // * (4*y-20)*(y-5)
(y-5)*(y-23/4) = 0
( y-23/4 )
y-23/4 = 0 // + 23/4
y = 23/4
( y-5 )
y-5 = 0 // + 5
y = 5
y in { 5}
y = 23/4
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