2 ∣ x ∣ ( 2 2 ∣ x ∣ − 1 6 ) ( 2 ∣ x ∣ − 8 ) ( 2 ∣ x ∣ + 1 ) ( 2 2 ∣ x ∣ + 2 ∣ x ∣ + 2 + 8 ) ≥ 0
Exactly how many integers do not satisfy the inequality above?
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Relevant wiki: Wavy Curve Method
Given x ∈ Z ⋯ ( 1 )
Let's put y = 2 ∣ x ∣ ⋯ ( 2 )
Let f ( x ) = 2 ∣ x ∣ ( 2 2 ∣ x ∣ − 1 6 ) ( 2 ∣ x ∣ − 8 ) ( 2 ∣ x ∣ + 1 ) ( 2 2 ∣ x ∣ + 2 ∣ x ∣ + 2 + 8 )
∣ x ∣ ≥ 0 ⇒ 2 ∣ x ∣ ≥ 2 0 = 1 ⇒ y ≥ 1 ⇒ y ∈ [ − 1 , ∞ ) ⋯ ( 3 )
So, f ( x ) = g ( y ) = y ( y 2 − 1 6 ) ( y − 8 ) ( y + 1 ) ( y 2 + 4 y + 8 )
We need g ( y ) ≥ 0 along with ( 1 ) , ( 2 ) and ( 3 )
Roots of expressions in g : y + 1 → − 1 y 2 + 4 y + 8 → no real roots y → 0 y 2 − 1 6 → − 4 , 4 y − 8 → 8 y = − 4 , 0 , 4 , 8
Effective degree of g is odd
Applying Wavy-Curve method ,
So, we get y ∈ ( − 4 , − 1 ] ∪ ( 0 , 4 ) ∪ ( 8 , ∞ )
Since y ≥ 1 , y ∈ [ 1 , 4 ) ∪ ( 8 , ∞ )
Integral values of y which are not satisfying are: y ∈ { 4 , 5 , 6 , 7 , 8 } ⇒ 2 ∣ x ∣ ∈ { 4 , 5 , 6 , 7 , 8 } , x ∈ Z ⇒ 2 ∣ x ∣ ∈ { 4 , 8 } x ∈ { − 3 , − 2 , 2 , 3 } 4 v a l u e s