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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
3 > 21/y // - 21/y
3-(21/y) > 0
3-21*y^-1 > 0
-21*y^-1 > -3
-21*y^-1 > -3 // * -1
21*y^-1 < 3
21*y^-1 < 3 // : 21
y^-1 < 3/21
y^-1 < 1/7
1/(y^1) < 1/7
y <> 0
1/(y^1) < 1/7 // * y^2
(y^2)/(y^1) < 1/7*y^2
y^1 < 1/7*y^2
y-1/7*y^2 < 0
y^1*(1-1/7*y) < 0
y < 0
-(abs(y))^1*(1/7*(abs(y))^1+1) < 0 // : -(abs(y))^1
1/7*(abs(y))^1+1 > 0
1/7*(abs(y))^1 > -1 // : 1/7
(abs(y))^1 > -1/1/7
(abs(y))^1 > -7 // ^ 1/1
abs(y) > (-7)^(1/1)
abs(y) > -7
y in (-oo:0)
y < 0
y > 0
y^1*(1-(1/7*y^1)) < 0 // : y^1
1-(1/7*y^1) < 0
1 < 1/7*y^1 // : 1/7
1/1/7 < y^1 // ^ 1/1
(1/1/7)^(1/1) < y
y in (7:+oo)
y > 0
y in (-oo:0) U (7:+oo)
y in (-oo:0) U (7:+oo)
(-oo:0) U (7:+oo)
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