MHB How to Solve Complex Algebraic Equations Involving Square Roots?

[abrk]

Hello.
I've been trying to crack these three for half a day and decided to ask for help.

1) $$(a+b(\sqrt{2})^2=33+20(\sqrt{2})$$

2) $$(a+b(\sqrt{2})^2=41-24(\sqrt{2})$$

3) Find: $$ \sqrt{11-6\sqrt{2}}$$.

 

[HallsofIvy]

Hello.
I've been trying to crack these three for half a day and decided to ask for help.

1) $$(a+b(\sqrt{2})^2=33+20(\sqrt{2})$$

2) $$(a+b(\sqrt{2})^2=41-24(\sqrt{2})$$

3) Find: $$ \sqrt{11-6\sqrt{2}}$$.

You are missing a ")" in the first two problems. Since just [math](\sqrt{2})^2[/math] would make the problem trivial I assume you mean [math](a+ b\sqrt{2})^2[/math] in each.

Okay, in those two just go ahead and square on the left. I presume you know that [math](x+ y)^2= x^2+ 2xy+ y^2[/math].
[math](a+ b \sqrt{2})^2= a^2+ 2ab\sqrt{2}+ 2b^2[/math]
So in (1) you have [math]a^2+ 2b^2+ 2ab\sqrt{2}= 33+ 20\sqrt{2}[/math]. Assuming that a and b are supposed to be rational numbers, you must have [math]a^2+ 2b= 33[/math] and [math]2ab= 20[/math]

For (3), I don't know what you mean by "find" that number. The way it is given is perfectly valid. I presume you do NOT mean just use a calculator!

 

[Petrus]

[math]a^2+ 2b= 33[/math]

Hello,
You forgot $$a^2+2b^2=33$$, just wanted to tell that you forgot that so you can Edit! Have a nice day!
Regards,
$$|\pi\rangle$$

 

[abrk]

You are missing a ")" in the first two problems. Since just [math](\sqrt{2})^2[/math] would make the problem trivial I assume you mean [math](a+ b\sqrt{2})^2[/math] in each.

Okay, in those two just go ahead and square on the left. I presume you know that [math](x+ y)^2= x^2+ 2xy+ y^2[/math].
[math](a+ b \sqrt{2})^2= a^2+ 2ab\sqrt{2}+ 2b^2[/math]
So in (1) you have [math]a^2+ 2b^2+ 2ab\sqrt{2}= 33+ 20\sqrt{2}[/math]. Assuming that a and b are supposed to be rational numbers, you must have [math]a^2+ 2b= 33[/math] and [math]2ab= 20[/math]

For (3), I don't know what you mean by "find" that number. The way it is given is perfectly valid. I presume you do NOT mean just use a calculator!

For 1 and 2. Indeed, there was one more bracket, which I didn't embed in my original question. My whole problem is that I cannot get through the equation system

$$a^2+b^2=c$$
$$a*b=d$$

Or in this case $$a^2+2b^2=33$$ and $$a*b=20$$

In 3, that's the whole description of the task, but a assume that it is to be represented as $$a+b\sqrt{2}$$, thus it is exactly the same as the first two problems.

 

[Petrus]

Hello,
we got that:
(1)$$a^2+2b^2=33$$
(2)$$a*b=20$$
from (2) we get that $$a=\frac{20}{b}$$ and put that into (1) and solve it!
Good luck and have a nice day!
Regards,
$$|\pi\rangle$$

 

[MarkFL]

For the 3rd problem, I would observe that:

$$11-6\sqrt{2}=3^2-2\cdot3\sqrt{2}+\sqrt{2}^2$$

Now it should be straightforward to factor this as the square of a surd.

 

[soroban]

Hello, abrk!

HallsofIvy gave you hints for the first two problems.

\text{3) Find: }\;\sqrt{11-6\sqrt{2}}

By now you may suspect that 11-6\sqrt{2} is a perfect square.

Let (a + b\sqrt{2})^2 \:=\:11-6\sqrt{2}
. . where a > 0 and a,b are rational numbers.

Then: .a^2 + 2b^2 + 2\sqrt{2}ab \:=\:11-6\sqrt{2}

Equate coefficients: .\begin{Bmatrix}a^2+2b^2 \:=\:11 & [1] \\ 2ab \:=\:\text{-}6 & [2] \end{Bmatrix}

From [2]: .b \,=\,\text{-}\tfrac{3}{a}\;\;[3]
Substitute into [1]: .a^2 + 2\left(\text{-}\tfrac{3}{a}\right)^2 \:=\:11

a^4 - 11a^2 + 18 \:=\:0 \quad\Rightarrow\quad (a^2-2)(a^2-9) \:=\:0

. . a \;=\;\pm\sqrt{2},\;\pm3

Hence: .\boxed{a \,=\,3}

Substitute into [3]: .b \,=\,\text{-}\tfrac{3}{3} \quad\Rightarrow\quad \boxed{b \,=\,\text{-}1}Hence: .(3-\sqrt{2})^2 \:=\:11 - 6\sqrt{2}

Therefore: .\sqrt{11-6\sqrt{2}} \;=\;3-\sqrt{2}

 

[abrk]

I didn't even suspect that level of help and good will is possible nowadays.

Thank you very very much.