0

Solve the following equation. $$(5+2\sqrt6)^{x^2-3}+(5-2\sqrt6)^{x^2-3} = 10$$

Solve for $x$ in this equation! I am stuck :/

ibuprofen
  • 525
  • 1
    See https://math.stackexchange.com/questions/1237837/solve-left-sqrt34-sqrt15-rightx-left-sqrt34-sqrt15-right?noredirect=1&lq=1 and https://math.stackexchange.com/questions/1330422/how-to-solve-this-equation-for-x-left-sqrt2-sqrt3-rightx-left-sq?noredirect=1&lq=1 – lab bhattacharjee Aug 18 '17 at 08:44
  • 1
    In order to get the best possible answers, it is helpful to state what your thoughts and attempts on the problem are; this will prevent people from telling you things you already know, and help them give their answers at the right level. – projectilemotion Aug 18 '17 at 08:45
  • @projectilemotion sure – ibuprofen Aug 18 '17 at 08:55

4 Answers4

4

Hint:

As $(5+2\sqrt6)(5-2\sqrt6)=1$

Method$\#1$:

set $(5+2\sqrt6)^{x^2-3}=a$ to find $$a+\dfrac1a=10$$

So, we have $$(5+2\sqrt6)^{x^2-3}=a=(5+2\sqrt6)^{\pm1}$$

Method$\#2$:

$$a+\dfrac1a=5+2\sqrt6+\dfrac1{5+2\sqrt6}$$

lab bhattacharjee
  • 274,582
3

Let $(5+2\sqrt6)^{x^2-3}=t$. Hence, $t+\frac{1}{t}=10$ and we have $t=5\pm2\sqrt{6}$.

Thus, $x^2-3=1$ or $x^2-3=-1$, which gives the answer: $$\{2,-2,\sqrt2,-\sqrt2\}$$

Michael Rozenberg
  • 194,933
2

$\left(5-2 \sqrt{6}\right) \left(5+2 \sqrt{6}\right)=1$

So $5-2 \sqrt{6}=\dfrac{1}{5+2 \sqrt{6}}$

substitute $\left(2 \sqrt{6}+5\right)^{x^2-3}=t$ so that $\left(5-2 \sqrt{6}\right)^{x^2-3}=\dfrac{1}{t}$

and solve $t+\dfrac{1}{t}=10$ which gives $t_1= 5-2 \sqrt{6},\;t_2=5+2 \sqrt{6}$

Substitute back

$\left(2 \sqrt{6}+5\right)^{x^2-3}=5-2 \sqrt{6}$

$\left(2 \sqrt{6}+5\right)^{x^2-3}=\left(5+2 \sqrt{6}\right)^{-1}$

$x^2-3=-1\to x=\pm\sqrt 2$

and

$\left(2 \sqrt{6}+5\right)^{x^2-3}=5+2 \sqrt{6}$

$x^2-3=1\to x=\pm 2$

Raffaele
  • 26,371
1

If you write $a = (5+2\sqrt6)^{x^2-3}$ then you have $a+1/a =10$ and thus $a_{1,2} = {5\pm 2\sqrt6}$.

So $x^2-3 = \pm 1$ and thus $x = \pm 2, \pm \sqrt{2}$.

nonuser
  • 90,026