Math Problem Statement

Let $z$ and $w$ be complex numbers such that $|z| = |w| = 1$ and $zw \ne -1.$

(a) Prove that $\overline{z} = \frac{1}{z}$ and $\overline{w} = \frac{1}{w}.$

Solution

To prove that z=1z\overline{z} = \frac{1}{z} and w=1w\overline{w} = \frac{1}{w}, let's break the problem down step by step for zz. The same reasoning will apply for ww.

Step 1: Use the property z=1|z| = 1

Given that z=1|z| = 1, recall that the modulus of a complex number z=a+biz = a + bi, where aa and bb are real, is defined as:

z=a2+b2|z| = \sqrt{a^2 + b^2}

However, since z=1|z| = 1, this implies:

a2+b2=1a2+b2=1\sqrt{a^2 + b^2} = 1 \quad \Rightarrow \quad a^2 + b^2 = 1

This tells us that the complex number zz lies on the unit circle in the complex plane.

Step 2: Conjugate of zz

The complex conjugate of z=a+biz = a + bi is:

z=abi\overline{z} = a - bi

Step 3: Inverse of zz

The inverse of z=a+biz = a + bi is given by:

1z=1a+bi\frac{1}{z} = \frac{1}{a + bi}

To simplify 1a+bi\frac{1}{a + bi}, we multiply both the numerator and the denominator by the complex conjugate of a+bia + bi, which is abia - bi. This yields:

1z=abi(a+bi)(abi)=abia2+b2\frac{1}{z} = \frac{a - bi}{(a + bi)(a - bi)} = \frac{a - bi}{a^2 + b^2}

Since we know a2+b2=1a^2 + b^2 = 1 (from Step 1), the expression simplifies to:

1z=abi=z\frac{1}{z} = a - bi = \overline{z}

Thus, we have shown that:

z=1z\overline{z} = \frac{1}{z}

Step 4: Apply the same reasoning to ww

Since w=1|w| = 1, we can follow exactly the same steps as above to show that:

w=1w\overline{w} = \frac{1}{w}

Conclusion

We have proven that:

z=1zandw=1w\overline{z} = \frac{1}{z} \quad \text{and} \quad \overline{w} = \frac{1}{w}

Would you like further clarification or details on any part?

Here are some related questions:

  1. How does the unit circle help in understanding complex numbers?
  2. Can you show a general proof for other moduli z1|z| \neq 1?
  3. How would this change if zz and ww were not on the unit circle?
  4. What other properties of complex numbers can be derived from z=1|z| = 1?
  5. How can this proof be extended to other forms of zz, such as polar form?

Tip: When dealing with complex numbers on the unit circle, it's often helpful to use their polar form, z=eiθz = e^{i\theta}, where computations involving modulus and conjugates become more straightforward.

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Math Problem Analysis

Mathematical Concepts

Complex Numbers
Conjugate
Inverse
Modulus

Formulas

|z| = \sqrt{a^2 + b^2}
\overline{z} = a - bi
\frac{1}{z} = \frac{a - bi}{a^2 + b^2}

Theorems

Modulus of a complex number
Properties of complex conjugates
Inverse of a complex number on the unit circle

Suitable Grade Level

Undergraduate Mathematics or Advanced High School