Math  /  Algebra

Question3. Prove that if zz and ww are complex numbers. d) zˉ1=(z1)\bar{z}^{-1}=\overline{\left(z^{-1}\right)} for z0z \neq 0.

Studdy Solution

STEP 1

What is this asking? We need to show that the inverse of the conjugate of zz is the same as the conjugate of the inverse of zz, assuming zz isn't zero. Watch out! Remember that the conjugate flips the sign of the imaginary part, and the inverse flips the complex number "upside down" in a special way.
Don't mix them up!

STEP 2

1. Define the complex number and its conjugate.
2. Compute the left-hand side.
3. Compute the right-hand side.
4. Compare the two sides.

STEP 3

Let's **define** our complex number zz as z=a+biz = a + bi, where aa and bb are real numbers, and ii is the imaginary unit.

STEP 4

The **conjugate** of zz, denoted as zˉ\bar{z}, is found by changing the sign of the imaginary part.
So, zˉ=abi\bar{z} = a - bi.

STEP 5

We want to compute zˉ1\bar{z}^{-1}.
First, let's find the inverse of zˉ\bar{z}.
We multiply zˉ\bar{z} by zˉˉzˉˉ\frac{\bar{\bar{z}}}{\bar{\bar{z}}}, which is just a fancy way of multiplying by one.

STEP 6

zˉ1=1zˉ=1abia+bia+bi=a+bia2(bi)2 \bar{z}^{-1} = \frac{1}{\bar{z}} = \frac{1}{a - bi} \cdot \frac{a + bi}{a + bi} = \frac{a + bi}{a^2 - (bi)^2} Remember, i2=1i^2 = -1, so (bi)2=b2(bi)^2 = -b^2.

STEP 7

zˉ1=a+bia2(b2)=a+bia2+b2=aa2+b2+ba2+b2i \bar{z}^{-1} = \frac{a + bi}{a^2 - (-b^2)} = \frac{a + bi}{a^2 + b^2} = \frac{a}{a^2 + b^2} + \frac{b}{a^2 + b^2}i So, we have our **left-hand side**!

STEP 8

Now, let's compute (z1)\overline{\left(z^{-1}\right)}.
First, we find z1z^{-1}.

STEP 9

z1=1z=1a+biabiabi=abia2+b2=aa2+b2ba2+b2i z^{-1} = \frac{1}{z} = \frac{1}{a + bi} \cdot \frac{a - bi}{a - bi} = \frac{a - bi}{a^2 + b^2} = \frac{a}{a^2 + b^2} - \frac{b}{a^2 + b^2}i

STEP 10

Now, we take the **conjugate** of this result: (z1)=aa2+b2ba2+b2i=aa2+b2+ba2+b2i \overline{\left(z^{-1}\right)} = \overline{\frac{a}{a^2 + b^2} - \frac{b}{a^2 + b^2}i} = \frac{a}{a^2 + b^2} + \frac{b}{a^2 + b^2}i And there's our **right-hand side**!

STEP 11

Let's compare the two sides: zˉ1=aa2+b2+ba2+b2i \bar{z}^{-1} = \frac{a}{a^2 + b^2} + \frac{b}{a^2 + b^2}i (z1)=aa2+b2+ba2+b2i \overline{\left(z^{-1}\right)} = \frac{a}{a^2 + b^2} + \frac{b}{a^2 + b^2}i They're the same!

STEP 12

We've shown that zˉ1=(z1)\bar{z}^{-1} = \overline{\left(z^{-1}\right)}!
We did this by carefully calculating both sides and seeing they match.

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