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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
4 = 7/y-1 // - 7/y-1
1-(7/y)+4 = 0
1-7*y^-1+4 = 0
-7*y^-1 = -5 // : -7
y^-1 = 5/7
-1 < 0
1/(y^1) = 5/7 // * y^1
1 = 5/7*y^1 // : 5/7
7/5 = y^1
y = 7/5
y = 7/5
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