Question

Let $z$ and $w$ be complex numbers satisfying $|z| = 4$ and $|w| = 2$. Then enter in the numbers\[|z+w|^2, |zw|^2, |z-w|^2, \left| \dfrac{z}{w} \right|^2 \]below, in the order listed above. If any of these cannot be uniquely determined from the information given, enter in a question mark.

Answer 1

a) |z+w|² cannot be uniquely determined from the information provided.

b) |zw|² = [|z| × |w|]² = (4×2)² = 64.

c) |z+w|² cannot be uniquely determined from the information provided.

d) |z/w|² = [|z|/|w|]² = (4/2)² = 4

Explanation:

z & w are complex numbers with magnitudes

|z| = 4

|w| = 2

We are the told to find

|z + w|²

|zw|²

|z - w|²

|z/w|²

Let the complex numbers be

z = x + iy

w = a + ib

|z| = √(x² + y²) = 4

|w| = √(a² + b²) = 2

|z|² = x² + y² = 16

|w|² = a² + b² = 4

z+w = (x + iy) + (a + ib) = (x + a) + i(y + b)

|z+w|² = (x + a)² + (y + b)² = x² + 2ax + a² + y² + 2by + b²

= (a² + b²) + (x² + y²) + 2ax + 2by

= |w|² + |z|² + 2ax + 2by

= 4 + 16 + 2ax + 2by

= 20 + 2(ax + by)

This cannot be determined from the information provided.

zw = (x + iy)(a + ib) = ax + i(bx + ay) - by

= (ax - by) + i(bx + ay)

|zw|² = a²x² + b²y² - 2abxy + b²x² + a²y² + 2abxy

= a²x² + b²y² + b²x² + a²y²

= a²(x² + y²) + b²(x² + y²)

= (a² + b²)(x² + y²)

= |w|² × |z|²

= 4×16

= 64

c) z-w = (x + iy) - (a + ib) = (x - a) + i(y - b)

|z-w|² = (x - a)² + (y - b)²

= x² - 2ax + a² + y² - 2by + b²

= (a² + b²) + (x² + y²) - 2ax - 2by

= |w|² + |z|² - 2ax - 2by

= 4 + 16 - 2ax - 2by

= 20 + 2(ax + by)

This cannot be determined from the information provided.

d) z/w = (x + iy)/(a + ib)

Rationalizing by multiplying numerator and denominator by (a - ib)

(z/w)= [(x + iy)(a - ib)/(a + ib)/(a - ib)]

= [ax - by + i(ay - bx)]/(a² + b²)

|z/w|² = [(ax + by)² + (ay - bx)²]/(a² + b²)²

= [a²x² + b²y² + 2abxy + b²x² + a²y² - 2abxy]/(a⁴ + b⁴ + 2a²b²)

= [a²x² + b²y² + b²x² + a²y²]/[(a² + b²)² - 2a²b² + 2a²b²]

= [(a² + b²)(x² + y²)]/[(a² + b²)²]

= [(x² + y²)/(a² + b²)]

= |z|²/|w|²

= (4/2)²

= 4

Hope this Helps!!!