What does it Equal? Part 2

Algebra Level 2

If you haven't seen part one yet, click here to see it.

If x 2 + x 2 = 3 x^2+x^{-2}=3 , then what are the possible values for x x ?

All possible values for x x can be written in the form ± a ± b c d \pm \sqrt {\dfrac{a \pm b{\sqrt {c}}}{d}} , where a a , b b , c c , and d d are the same coprime positive integers for all the value of x x , but the ± \pm signs can alter between + + and - . What is a + b + c + d + 2 a+b+c+d+2 ?


The answer is 13.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Jesse Li
Dec 11, 2018

x 2 + x 2 = 3 x^2+x^{-2}=3

x 2 + 1 x 2 = 3 x 2 = q x^2+\frac{1}{x^2}=3 \rightarrow x^2=q

q + 1 q = 3 q+\frac{1}{q}=3

q 3 + 1 q = 0 q-3+\frac{1}{q}=0

q 2 3 q + 1 = 0 q^2-3q+1=0

q = 3 ± 5 2 q=\frac{3 \pm \sqrt{5}}{2}

x = ± 3 ± 5 2 x=\pm \sqrt{\frac{3 \pm \sqrt{5}}{2}}

a = 3 a=3

b = 1 b=1

c = 5 c=5

d = 2 d=2

3 + 1 + 5 + 2 + 2 = 13 3+1+5+2+2=\boxed{13}

Chew-Seong Cheong
Dec 12, 2018

x 2 + x 2 = 3 x 2 + 1 x 2 = 3 Add 2 on both sides. x 2 2 + 1 x 2 = 1 ( x 1 x ) 2 = 1 Take square root on both sides. x 1 x = ± 1 \begin{aligned} x^2 + x^{-2} & = 3 \\ x^2 + \frac 1{x^2} & = 3 & \small \color{#3D99F6} \text{Add } -2 \text{ on both sides.} \\ x^2 - 2 + \frac 1{x^2} & = 1 \\ \left(x - \frac 1x\right)^2 & = 1 & \small \color{#3D99F6} \text{Take square root on both sides.} \\ x - \frac 1x & = \pm 1 \end{aligned}

{ x 2 x 1 = 0 { x = 1 + 5 2 = ( 1 + 5 2 ) 2 = 3 + 5 2 x = 1 5 2 = ( 5 1 2 ) 2 = 3 5 2 x 2 + x 1 = 0 { x = 1 + 5 2 = ( 5 1 2 ) 2 = 3 5 2 x = 1 5 2 = ( 1 + 5 2 ) 2 = 3 + 5 2 \implies \begin{cases} x^2 - x - 1 & = 0 & \implies \begin{cases} x = \dfrac {1+\sqrt 5}2 = \sqrt{\left(\dfrac {1+\sqrt 5}2 \right)^2} = \sqrt{\dfrac {3+\sqrt 5}2} \\ x = \dfrac {1-\sqrt 5}2 = - \sqrt{\left(\dfrac {\sqrt 5-1}2 \right)^2} = - \sqrt{\dfrac {3-\sqrt 5}2} \end{cases} \\ x^2 + x - 1 & = 0 & \implies \begin{cases} x = \dfrac {-1+\sqrt 5}2 = \sqrt{\left(\dfrac {\sqrt 5-1}2 \right)^2} = \sqrt{\dfrac {3-\sqrt 5}2} \\ x = \dfrac {-1-\sqrt 5}2 = - \sqrt{\left(\dfrac {1+\sqrt 5}2 \right)^2} = - \sqrt{\dfrac {3+\sqrt 5}2} \end{cases} \end{cases}

Therefore, a + b + c + d + 2 = 3 + 1 + 5 + 2 + 2 = 13 a+b+c+d+2 = 3+1+5+2+2 = \boxed{13} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...