Substitution is definitely useful here

Algebra Level 4

( 2 x + 1 ) ( 2 2 x + 2 x + 2 + 8 ) 2 x ( 2 2 x 16 ) ( 2 x 8 ) 0 \frac{(2^{|x|}+1)(2^{2|x|}+2^{|x|+2}+8)}{2^{|x|} (2^{2|x|}-16)(2^{|x|}-8)}≥0

Exactly how many integers do not satisfy the inequality above?


The answer is 4.

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1 solution

Relevant wiki: Wavy Curve Method

Given x Z ( 1 ) x\in\mathbb Z~\cdots (1)

Let's put y = 2 x ( 2 ) y=2^{|x|}~\cdots (2)

Let f ( x ) = ( 2 x + 1 ) ( 2 2 x + 2 x + 2 + 8 ) 2 x ( 2 2 x 16 ) ( 2 x 8 ) f(x) = \dfrac{(2^{|x|}+1)(2^{2|x|}+2^{|x|+2}+8)}{2^{|x|} (2^{2|x|}-16)(2^{|x|}-8)}

x 0 2 x 2 0 = 1 y 1 y [ 1 , ) ( 3 ) |x|\ge0\\\Rightarrow 2^{|x|}\ge 2^0=1\\\Rightarrow y\ge1\\\Rightarrow y\in\left[\left.-1,\infty\right.\right)~\cdots (3)

So, f ( x ) = g ( y ) = ( y + 1 ) ( y 2 + 4 y + 8 ) y ( y 2 16 ) ( y 8 ) f(x) = g(y) = \frac{(y+1)(y^2+4y+8)}{y (y^2-16)(y-8)}

We need g ( y ) 0 g(y)\ge 0 along with ( 1 ) , ( 2 ) (1), ~(2) and ( 3 ) (3)

Roots of expressions in g g : y + 1 1 y 2 + 4 y + 8 no real roots y 0 y 2 16 4 , 4 y 8 8 y 4 , 0 , 4 , 8 y+1\rightarrow -1\\y^2+4y+8\rightarrow \text{no real roots}\\y\rightarrow 0 \\ y^2-16\rightarrow -4,~4\\y-8\rightarrow 8\\ y\ne -4, 0, 4, 8

Effective degree of g g is odd

Applying Wavy-Curve method ,

So, we get y ( 4 , 1 ] ( 0 , 4 ) ( 8 , ) y \in (-4, -1]\cup(0,4)\cup(8,\infty)

Since y 1 y\ge1 , y [ 1 , 4 ) ( 8 , ) y \in [1,4)\cup (8,\infty)

Integral values of y 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 y \in \{4,5,6,7,8\}\\\Rightarrow 2^{|x|}\in \{4,5,6,7,8\}, x\in \mathbb Z\\\Rightarrow 2^{|x|} \in \{4,8\}\\\boxed{x \in \{-3, -2, 2, 3\}}\\ \boxed{4~ \mathrm{ values}}

Nice solution! :)

Wee Xian Bin - 4 years, 7 months ago

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Thank you!

Kishore S. Shenoy - 4 years, 7 months ago

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