Distance Midpoints in the Complex Plane

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Algebra 2 › Distance Midpoints in the Complex Plane

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1

Let $z_1=-6+4i$ and $z_2=2-2i$ be points on the complex plane. Using the midpoint formula $$\frac{z_1+z_2}{2}=\left(\frac{a+c}{2}\right)+\left(\frac{b+d}{2}\right)i,$$ what is the midpoint of the segment joining $z_1$ and $z_2$?

$-2+3i$

$-2+i$

$-4+2i$

$-8+2i$

Explanation

This question tests your understanding of finding midpoints between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The midpoint formula is: midpoint = (z₁ + z₂)/2 = ((a + c)/2) + ((b + d)/2)i. To find the midpoint of z₁ = -6 + 4i and z₂ = 2 - 2i: (1) Add the complex numbers: (-6 + 4i) + (2 - 2i) = -4 + 2i. (2) Divide by 2: (-4 + 2i)/2 = -2 + i. The midpoint is at -2 + i, exactly halfway between the two points. Choice A correctly applies the midpoint formula. Choice B shows -4 + 2i, which is the sum without dividing by 2—remember the averaging step! Choice D shows -8 + 2i, which might come from multiplying instead of adding. Always add the complex numbers first, THEN divide both real and imaginary parts by 2. The midpoint formula works because you're essentially averaging the x-coordinates (real parts) and y-coordinates (imaginary parts) separately.

2

Treat $z=a+bi$ as the point $(a,b)$ on the complex plane. Using the midpoint formula

$$\frac{z_1+z_2}{2}=\frac{a+c}{2}+\frac{b+d}{2}i,$$

what is the midpoint of the segment joining $z_1=-1+3i$ and $z_2=5+i$?

$4+4i$

$2+2i$

$2+i$

$-3+2i$

Explanation

This question tests your understanding of finding midpoints between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The midpoint formula for complex numbers is midpoint = (z₁ + z₂)/2 = [(a + c)/2] + [(b + d)/2]i, averaging the real and imaginary parts separately. To find the midpoint of -1 + 3i and 5 + i: (1) Add: (-1 + 3i) + (5 + i) = 4 + 4i. (2) Divide by 2: (4 + 4i)/2 = 4/2 + 4i/2 = 2 + 2i. The midpoint is at 2 + 2i, equidistant from both original points! Choice A correctly applies the midpoint formula. Choice B shows 4 + 4i, the sum without dividing—always remember to divide by 2 to get the midpoint, not just the sum! The midpoint formula essentially averages the x-coordinates (real parts) and y-coordinates (imaginary parts) separately, just like in coordinate geometry.

3

Points $z_1=a+bi$ and $z_2=c+di$ on the complex plane are separated by distance $|z_1-z_2|=\sqrt{(a-c)^2+(b-d)^2}$, the same as the 2D distance formula. What is the distance between $z_1=5+2i$ and $z_2=1-3i$?

$4-5i$

$\sqrt{41}$

$\sqrt{20}$

$\sqrt{9}$

Explanation

This question tests your understanding of finding distances between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The distance between z₁ = a + bi and z₂ = c + di is |z₁ - z₂| = √[(a - c)² + (b - d)²], exactly like the 2D distance formula. To find the distance from 5 + 2i to 1 - 3i: (1) Calculate difference: (5 + 2i) - (1 - 3i) = 5 + 2i - 1 + 3i = 4 + 5i. (2) Find modulus: |4 + 5i| = √(4² + 5²) = √(16 + 25) = √41. Choice A correctly applies the distance formula to get √41. Choice C shows 4 - 5i, which appears to be the difference with a sign error on the imaginary part—when subtracting 1 - 3i, remember that -(-3i) = +3i, so 2i - (-3i) = 2i + 3i = 5i! Always be careful with signs when subtracting complex numbers, especially when the second number has negative parts.

4

Let $z_1=2+3i$ and $z_2=8+1i$ represent points on the complex plane. The midpoint is $\dfrac{z_1+z_2}{2}=\left(\dfrac{a+c}{2}\right)+\left(\dfrac{b+d}{2}\right)i$, matching the coordinate midpoint formula. What is the midpoint?

$10+4i$

$3+1i$

$5+1i$

$5+2i$

Explanation

This question tests your understanding of finding midpoints between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The midpoint formula averages the coordinates: midpoint = (z₁ + z₂)/2 = ((a + c)/2) + ((b + d)/2)i, matching the coordinate midpoint formula exactly! To find the midpoint of z₁ = 2 + 3i and z₂ = 8 + 1i: (1) Add the complex numbers: (2 + 3i) + (8 + 1i) = (2 + 8) + (3 + 1)i = 10 + 4i. (2) Divide by 2: (10 + 4i)/2 = 10/2 + 4i/2 = 5 + 2i. The midpoint is at 5 + 2i, perfectly centered between the two points! Choice A correctly shows 5 + 2i as the midpoint. Choice B shows 10 + 4i, which is the sum before dividing - that's a common error where students forget the averaging step! The complex plane midpoint formula gives us the exact center of the line segment connecting two complex numbers.

5

On the complex plane, the point $z=a+bi$ corresponds to $(a,b)$. The distance between $z_1=a+bi$ and $z_2=c+di$ is $|z_1-z_2|=\sqrt{(a-c)^2+(b-d)^2$, the same as the coordinate distance formula. What is the distance between $z_1=3+2i$ and $z_2=1-i$?

$\sqrt{25}$

$2+3i$

$\sqrt{13}$

$\sqrt{5}$

Explanation

This question tests your understanding of finding distances between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The distance between two complex numbers z₁ = a + bi and z₂ = c + di is the modulus of their difference: distance = |z₁ - z₂| = √((a - c)² + (b - d)²), which is exactly the 2D distance formula because the complex plane is a coordinate system! To find the distance from 3 + 2i to 1 - i: (1) Calculate difference: (3 + 2i) - (1 - i) = 2 + 3i. (2) Find modulus: |2 + 3i| = √(2² + 3²) = √(4 + 9) = √13. (3) The distance is √13 units—great job applying the formula! Choice C correctly applies the distance formula as the modulus of the difference. Choice B calculates the difference without taking the modulus: 2 + 3i is a vector, not the distance—you must find its length to get the scalar distance! For transferable strategy, always subtract the complex numbers, then take the modulus of the result: for example, distance from 4 + i to 2 - 3i is |(4 + i) - (2 - 3i)| = |2 + 4i| = √(4 + 16) = √20 = 2√5, symmetric regardless of subtraction order. Keep practicing these, and you'll master visualizing complex numbers geometrically—you've got this!

6

Let $z_1=-6+2i$ and $z_2=2-4i$ be points on the complex plane. The midpoint is $\dfrac{z_1+z_2}{2}$ (averaging real and imaginary parts, as in coordinate geometry). What is the midpoint?

$-4-2i$

$-8-2i$

$-2-i$

$-2-2i$

Explanation

This question tests your understanding of finding midpoints between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The midpoint formula works by averaging: midpoint = (z₁ + z₂)/2 = ((a + c)/2) + ((b + d)/2)i, averaging the real and imaginary parts separately. To find the midpoint of z₁ = -6 + 2i and z₂ = 2 - 4i: (1) Add the complex numbers: (-6 + 2i) + (2 - 4i) = (-6 + 2) + (2 - 4)i = -4 - 2i. (2) Divide by 2: (-4 - 2i)/2 = -4/2 - 2i/2 = -2 - i. The midpoint is at -2 - i, exactly halfway between the two points! Choice A correctly shows -2 - i as the midpoint. Choice B shows -4 - 2i, which is the sum before dividing by 2 - don't forget to average! The midpoint formula for complex numbers gives the same result as plotting the points on a coordinate plane and finding the midpoint geometrically.

7

On the complex plane, the distance between $z_1=a+bi$ and $z_2=c+di$ is the modulus of their difference: $|z_1-z_2|=\sqrt{(a-c)^2+(b-d)^2}$. What is the distance between $z_1=-2-i$ and $z_2=3+2i$?

$\sqrt{25}$

$\sqrt{16}$

$\sqrt{34}$

$5+3i$

Explanation

This question tests your understanding of finding distances between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The distance between two complex numbers z₁ = a + bi and z₂ = c + di is the modulus of their difference: distance = |z₁ - z₂| = √[(a - c)² + (b - d)²]. To find the distance from -2 - i to 3 + 2i: (1) Calculate difference: (-2 - i) - (3 + 2i) = -2 - i - 3 - 2i = -5 - 3i. (2) Find modulus: |-5 - 3i| = √[(-5)² + (-3)²] = √(25 + 9) = √34. Choice A correctly applies the distance formula, giving √34. Choice B shows 5 + 3i, which might come from incorrectly handling the signs when subtracting—remember that (-2) - 3 = -5 and (-1) - 2 = -3, but the modulus squares these values so signs don't affect the final distance! Distance is always positive because we square the components before taking the square root.

8

On the complex plane, the point $z=a+bi$ corresponds to $(a,b)$. The distance between $z_1=a+bi$ and $z_2=c+di$ is $|z_1-z_2|=\sqrt{(a-c)^2+(b-d)^2}$ (the same as the coordinate geometry distance formula). What is the distance between $z_1=3+2i$ and $z_2=1-i$?

$\sqrt{13}$

$5$

$\sqrt{5}$

$2+3i$

Explanation

This question tests your understanding of finding distances between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The distance between two complex numbers $z_1 = a + bi$ and $z_2 = c + di$ is the modulus of their difference: distance = $|z_1 - z_2| = \sqrt{(a - c)^2 + (b - d)^2}$, which is exactly the 2D distance formula because the complex plane is a coordinate system! The midpoint formula also works the same: midpoint = $(z_1 + z_2)/2 = ((a + c)/2) + ((b + d)/2)i$, averaging the real and imaginary parts separately—these formulas connect complex number arithmetic with geometry beautifully. To find the distance from $3 + 2i$ to $1 - i$: (1) Calculate difference: $(3 + 2i) - (1 - i) = 2 + 3i$; (2) Find modulus: $|2 + 3i| = \sqrt{2^2 + 3^2} = \sqrt{4 + 9} = \sqrt{13}$; (3) So the distance is $\sqrt{13}$ units—geometrically, plot both points and the distance is the straight-line length between them! Choice B correctly applies the distance formula as the modulus of the difference. Choice D calculates the difference without taking the modulus: $2 + 3i$ is a vector, not the distance—you must find its length (modulus) to get the actual distance, so always take the square root after squaring and adding! Remember the distance formula for complex numbers: (1) Subtract the complex numbers (order doesn't matter for distance); (2) Find the modulus of the result: if the difference is $p + qi$, modulus = $\sqrt{p^2 + q^2}$; (3) That's the distance—keep practicing, you've got this! For example, distance from $4 + i$ to $2 - i$: difference = $2 + 2i$, modulus = $\sqrt{4 + 4} = \sqrt{8} = 2\sqrt{2}$, and it's symmetric if you subtract the other way.

9

On the complex plane, $z=a+bi$ corresponds to the point $(a,b)$. For $z_1=-1+4i$ and $z_2=3$, which complex number is the midpoint of the segment connecting them, using $\dfrac{z_1+z_2}{2}$?

$-2+2i$

$1+4i$

$2+4i$

$1+2i$

Explanation

This question tests your understanding of finding midpoints between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The midpoint formula works the same: midpoint = (z₁ + z₂)/2 = ((a + c)/2) + ((b + d)/2)i, averaging the real parts and imaginary parts separately—these formulas connect complex number arithmetic with geometry beautifully. For the midpoint of -1 + 4i and 3 + 0i: (1) Add: (-1 + 4i) + (3 + 0i) = 2 + 4i; (2) Divide by 2: (2 + 4i)/2 = 1 + 2i, exactly halfway between the two points on the complex plane—geometrically, plot both points and the midpoint lies on the line segment connecting them, equidistant from each! Choice A correctly applies the midpoint formula as the average of the two complex numbers. Choice B makes an arithmetic error by adding instead of averaging: (-1 + 4i) + (3 + 0i) = 2 + 4i, but you must divide the entire result by 2—midpoint formula: add both complex numbers first, then divide by 2, averaging both real and imaginary parts. Midpoint formula: (1) Add the two complex numbers: (a + bi) + (c + di) = (a + c) + (b + d)i; (2) Divide result by 2: real part (a + c)/2, imaginary part (b + d)/2; (3) Write as ((a + c)/2) + ((b + d)/2)i—essentially, average x-coordinates for real part, average y-coordinates for imaginary part, just like coordinate midpoint! For 4 + 2i and 2 - 6i: add to get 6 - 4i, divide by 2 to get 3 - 2i as midpoint—the complex plane formulas are identical to coordinate geometry because the complex plane is a coordinate system, and great job tackling these!

10

On the complex plane, the difference $z_1-z_2$ represents the vector from $z_2$ to $z_1$, and its modulus gives the distance: $|z_1-z_2|=\sqrt{(a-c)^2+(b-d)^2}$. What is the distance between $z_1=2+3i$ and $z_2=-1+7i$?

$\sqrt{49}$

$\sqrt{7}$

$3+4i$

$\sqrt{25}$

Explanation

This question tests your understanding of finding distances between complex numbers on the complex plane using formulas that mirror 2D coordinate geometry. The distance is |z₁ - z₂| = √[(a - c)² + (b - d)²]. To find the distance from 2 + 3i to -1 + 7i: (1) Calculate difference: (2 + 3i) - (-1 + 7i) = 2 + 3i + 1 - 7i = 3 - 4i. (2) Find modulus: |3 - 4i| = √(3² + (-4)²) = √(9 + 16) = √25 = 5. Choice B correctly gives √25. Choice C shows 3 + 4i, which has the wrong sign on the imaginary part—when subtracting -1 + 7i, we get 3i - 7i = -4i, not +4i! The distance formula gives the same result regardless of which point you subtract from which, as long as you correctly find the modulus of the difference.

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