Easy quadratic problem

Algebra Level 3

We call ‘p’ a good number if the inequality 2 x 2 + 2 x + 3 x 2 + x + 1 \dfrac{2x^2+2x+3}{x^2+x+1} is smaller than or equal to p for any integral x. Find the smallest integral good number


The answer is 3.

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2 solutions

Tom Engelsman
Feb 16, 2019

Knowing that 2 x 2 + 2 x + 3 x 2 + x + 1 = 2 + 1 x 2 + x + 1 \frac{2x^2 + 2x + 3}{x^2 + x + 1} = 2 + \frac{1}{x^2+x+1} , we require (x^2 + x + 1)|1 for x R x \in \mathbb{R} . Hence:

x 2 + x + 1 = ± 1 x^2 + x + 1 = \pm1 .

If it's equal to 1, then x 2 + x = 0 x = 0 , 1 x^2 + x = 0 \Rightarrow x = 0,-1 . If it's equal to -1, then x 2 + x + 2 = 0 x = 1 ± 7 i 2 x^2 + x + 2 = 0 \Rightarrow x = \frac{-1 \pm \sqrt{7}i}{2} and we do not have real roots! Therefore the least integral value of p p such that 2 + 1 x 2 + x + 1 p 2 + \frac{1}{x^2+x+1} \le p holds is 3 . \boxed{3}.

My approach was same but some people...in my friend circle and on brilliant say that answer is 4

Miss Physicist - 2 years, 3 months ago

For x=-0.5 the fraction takes value 10/3 which is larger than 3, but since we were asked for integral (integer) and 4 is not correct its got to be 3. NOT the best problem tho..

Eric Scholz - 2 years, 3 months ago
Amal Hari
Feb 16, 2019

2 [ x 2 + x + 1 ] + 1 [ x 2 + x + 1 ] \frac{2[x^{2} +x +1] +1}{[x^{2} +x +1]} , Let the polynomial be X X

2 X + 1 X \frac{2X +1}{X} = 2 + 1 X 2+\frac{1}{X}

2 + 1 X p 2+\frac{1}{X} \leq p

Lowest possible answer to this inequality is 2 when X approach \infty

Since we need integral value that is defined, taking limits is not an option because 2 is never reached and since the polynomial doesn't take negative values the only possible answer here is 3.

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