Solve for m

Algebra Level 3

If the inequality m x 2 + 3 x + 4 x 2 + 2 x + 2 < 5 \dfrac{mx^2+3x+4}{x^2+2x+2} < 5 is true for all real x x , find the maximum integer value of m m .


The answer is 2.

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2 solutions

Adhiraj Dutta
Jan 26, 2020

Given, m x 2 + 3 x + 4 x 2 + 2 x + 2 < 5 \frac{mx^2 + 3x + 4}{x^2 + 2x + 2} < 5

m x 2 + 3 x + 4 < 5 ( x 2 + 2 x + 2 ) \implies mx^2+3x+4<5(x^2+2x+2)

( m 5 ) x 2 7 x 6 < 0 \implies (m-5)x^2-7x-6<0 , x R \forall x \in R

\therefore Discriminant < 0 < 0

49 + 24 ( m 5 ) < 0 \implies 49 + 24(m-5)<0

m < 71 24 \implies m<\frac{71}{24}

m 2 \therefore m \leq 2

Chew-Seong Cheong
Jan 27, 2020

m x 2 + 3 x + 4 x 2 + 2 x + 2 < 5 m x 2 + 3 x + 4 < 5 x 2 + 10 x + 10 ( m 5 ) x 2 7 x 6 < 0 7 2 4 ( m 5 ) ( 6 ) < 0 24 m < 71 m < 71 24 2.958333 \begin{aligned} \frac {mx^2+3x+4}{x^2+2x+2} & < 5 \\ mx^2 + 3x + 4 & < 5x^2 + 10 x + 10 \\ (m-5)x^2 - 7x - 6 & < 0 \\ \implies 7^2 - 4(m-5)(-6) & < 0 \\ 24 m & < 71 \\ m & < \frac {71}{24} \approx 2.958333 \end{aligned}

Therefore the maximum integer m m is 2 \boxed 2 .

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