TheXSquaredFactor

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UsernameTheXSquaredFactor
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Questions 3
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 #2
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Understandably, square roots cause a lot of confusion. This is one prime example of this occurring. To be clear, by definition, \(i=\sqrt{-1}\). When you calculate the square root of a number, it is implied that the output is the principal root or the positive result. 

 

For example, 

\(\sqrt{4}=2\\ \sqrt{9}=3\)

 

However, if you place a negative sign before the square root, then it is implied that the output should be negative. 

 

\(-\sqrt{4}=-2\\ -\sqrt{9}=-3\)

 

If a plus-minus sign precedes a square root, then both answers are desired:

 

\(\pm\sqrt{4}=\pm2\\ \pm\sqrt{9}=\pm3\)

 

This confuses many people, and it leads to misconceptions. Since \(\sqrt{-1}\) implies the principal root, the only answer is \(i\).

 

Examine the false proof that your "guy" provided, which seems to prove that i equals plus or minus 1 to the untrained eye.

 

\(i^2\\ i^\frac{4}{2}\\ \left(i^4\right)^{0.5}\\ i^{(2+2)^{0.5}}\\ \textcolor{red}{\left(i^2*i^2\right)^{0.5}}\)

 

The step highlighted in red in an invalid step. It ignores the part about exponent towers. In line 4, you have to evaluate the innermost exponent first to abide by order of operations. In line 5, there are now parentheses around the \(i^2*i^2\), which changes the overall value of the expression.  If this is unclear for you, maybe I will try a simpler example. 

 

Consider the following two expressions \(2^{3^2}\text{ and }\left(2^3\right)^2\)? The expressions here have a subtle difference, but the output is different. You compute these differently, too. This is another area where misconceptions are prevalent.

 

\(2^{\textcolor{red}{3^2}}=2^9=512\\ \textcolor{red}{\left(2^3\right)}^2=8^2=64\)

 

Notice that in the second expression that the parentheses take precedence over any other operation, so you must do that first. Now, let's relate that to the false proof.

 

\(i^{(2+2)^{0.5}}=\textcolor{red}{i^{4^{0.5}}}=i^2=-1\\ (i^2*i^2)^{0.5}=(i^{2+2})^{0.5}=\textcolor{red}{\left(i^4\right)^{0.5}}\)

 

Do you notice the discrepancy now? You can now see that the values are different. Since I have cleared up that confusion, I will now end the debate about which complex number the product simplifies to. I'll jump right to the moment of the confusion, which occurs at \(18+3i-3i^2\)

 

\(18+3i-3i^2\)By definition, \(i=\sqrt{-1}\), so squaring both sides would mean that \(i^2=-1\).
\(18+3i-3*-1\)Now, simplify as much as possible. 
\(18+3i+3\)Combine the like terms. 
\(21+3i\)This is completely simplified, and nothing more can be done here.
  
 #7
avatar+2441 
+3

Well, trial and error appeared to be my friend here. If I was not aware that the original expression could be simplified further, I probably would have made the same conclusion as you did, Melody. It is considered improper to have radicals or fractional exponents in the denominator, so I knew that there was some way to simplify this. My method is really only relevant to this particular problem. You will see why. 

 

Now, let's consider that extra bit on the end, the \(9^\frac{1}{3}=\left(3^2\right)^\frac{1}{3}=3^\frac{2}{3}=3^{\left(\frac{1}{3}\right)^2}\). This means that the denominator can be written like \(1+3^\frac{1}{3}+3^{\left(\frac{1}{3}\right)^2}\), and if I let \(x=3^\frac{1}{3}\), then I can represent the denominator like \(x^2+x+1\). I then made a stunning realization. This required some extraordinary observational skills. 

 

\(x^3-y^3=(x-y)(x^2+xy+y^2)\) I am sure that you are familiar with this. It's the factorization of a difference of cubes. Let's set y equal to 1 and substitute.
\(x^3-1=(x-1)(x^2+x+1)\) Wait a second! Look at what one of the factors is. It's \(x^2+x+1\), which is also the expression of the denominator. This tells me that  if I multiply the trinomial in the denominator by x-1, then I will be left with two terms. This is perfect! I have now found a way to manipulate the denominator into two terms. Let's keep going. Since \(x=3^\frac{1}{3}\), let's just substitute it in. 
\(3^{\left(\frac{1}{3}\right)^3}-1=\left(3^\frac{1}{3}-1\right)\left(3^{\left(\frac{1}{3}\right)^2}+3^\frac{1}{3}+1\right)\) Let's complete the simplification here. The process is quite straightforward when you are comfortable with the law of indices. 
\(3^{\left(\frac{1}{3}\right)^3}-1\\ 3^{\frac{1}{3}*3}-1\\ 3^1-1\\ 3-1\\ 2\) Of course, notice what I multiplied the denominator by in the original problem. It is \(3^\frac{1}{3}-1\).
   

 

So, no, the conjugate is not "magical." The only issue with this thought process is that this will not help you on most types of problems with multi-term denominators. This is just one particular case, and I happened to crack it.