Is it Quadratic or not?

Algebra Level 4

If the equation x 4 x 2 + 1 = k x^4 - x^2 + 1 = k , where k k is a real number, has all real roots then what is the number of integral values of 8 k 8k ?


The answer is 3.

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1 solution

Chew-Seong Cheong
Nov 24, 2015

x 4 x 2 + 1 = k x 4 x 2 + 1 k = 0 This is a quadratic equation of x 2 x 2 = 1 ± 1 4 + 4 k 2 = 1 ± 4 k 3 2 \begin{aligned} x^4-x^2+1 & = k \\ x^4-x^2+1 - k & = 0 \quad \quad \quad \quad \quad \quad \quad \quad \small \color{#3D99F6}{\text{This is a quadratic equation of }x^2} \\ \Rightarrow x^2 & = \frac{1 \pm \sqrt{1-4+4k}}{2}\\ & = \frac{1 \pm \sqrt{4k-3}}{2} \end{aligned}

For real roots, { 4 k 3 0 k 3 4 1 4 k 3 2 0 1 4 k 3 0 4 k 3 1 4 k 3 1 k 1 \begin{cases} 4k-3 \ge 0 & \implies k \ge \dfrac{3}{4} \\ \dfrac{1 - \sqrt{4k-3}}{2} \ge 0 & \implies 1 - \sqrt{4k-3} \ge 0 & \implies \sqrt{4k-3} \le 1 & \implies 4k-3 \le 1 & \implies k \le 1 \end{cases}

3 4 k 1 6 8 k 8 \begin{aligned} \implies \frac{3}{4} \le k & \le 1 \implies 6 \le 8k & \le 8 \implies \end{aligned} There are 3 \boxed{3} integer values of 8 k 8k , which are 6 6 , 7 7 and 8 8 .

A slight mistake on the "k being greater or equal to 1" part.

Shouldn't it be "k being lesser or equal to 1"?

Kok Hao - 3 years, 5 months ago

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Thanks, I have changed it.

Chew-Seong Cheong - 3 years, 5 months ago

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