Distance and Midpoint Formulas

Working with coordinates in the plane

Distance and Midpoint Formulas

Distance Formula

The distance between two points (x1,y1)(x_1, y_1) and (x2,y2)(x_2, y_2):

d=(x2x1)2+(y2y1)2d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}

Derivation: This comes from the Pythagorean Theorem!

Midpoint Formula

The midpoint between two points (x1,y1)(x_1, y_1) and (x2,y2)(x_2, y_2):

M=(x1+x22,y1+y22)M = \left(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\right)

Memory aid: Average the x-coordinates, average the y-coordinates.

Applications

Perimeter: Add distances between consecutive vertices

Proving shapes:

  • Square: All 4 sides equal, diagonals equal
  • Rectangle: Opposite sides equal, diagonals equal
  • Rhombus: All 4 sides equal
  • Isosceles triangle: Two sides equal

Segment Partitioning

To find a point that divides a segment in ratio m:nm:n:

Use weighted average based on the ratio

Coordinate Proof Strategy

  1. Place figure on coordinate plane strategically
  2. Use distance/midpoint formulas
  3. Show required properties

📚 Practice Problems

1Problem 1easy

Question:

Find the distance between points A(3, 4) and B(7, 1).

💡 Show Solution

Step 1: Recall the distance formula: d = √[(x₂ - x₁)² + (y₂ - y₁)²]

Step 2: Identify the coordinates: Point A: (x₁, y₁) = (3, 4) Point B: (x₂, y₂) = (7, 1)

Step 3: Substitute into the formula: d = √[(7 - 3)² + (1 - 4)²] d = √[4² + (-3)²] d = √[16 + 9] d = √25 d = 5

Answer: The distance is 5 units

2Problem 2easy

Question:

Find the distance between points (3,4)(3, 4) and (7,1)(7, 1).

💡 Show Solution

Use the distance formula: d=(x2x1)2+(y2y1)2d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}

d=(73)2+(14)2d = \sqrt{(7 - 3)^2 + (1 - 4)^2}

d=42+(3)2d = \sqrt{4^2 + (-3)^2}

d=16+9d = \sqrt{16 + 9}

d=25=5d = \sqrt{25} = 5

Answer: 5 units

3Problem 3easy

Question:

Find the midpoint of the segment connecting (-2, 5) and (6, -3).

💡 Show Solution

Step 1: Recall the midpoint formula: M = ((x₁ + x₂)/2, (y₁ + y₂)/2)

Step 2: Identify the coordinates: Point 1: (x₁, y₁) = (-2, 5) Point 2: (x₂, y₂) = (6, -3)

Step 3: Calculate x-coordinate of midpoint: x_m = (-2 + 6)/2 = 4/2 = 2

Step 4: Calculate y-coordinate of midpoint: y_m = (5 + (-3))/2 = 2/2 = 1

Step 5: Write the midpoint: M = (2, 1)

Answer: The midpoint is (2, 1)

4Problem 4medium

Question:

Find the midpoint of the segment connecting (2,5)(-2, 5) and (6,3)(6, -3).

💡 Show Solution

Use the midpoint formula: M=(x1+x22,y1+y22)M = \left(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\right)

M=(2+62,5+(3)2)M = \left(\frac{-2 + 6}{2}, \frac{5 + (-3)}{2}\right)

M=(42,22)M = \left(\frac{4}{2}, \frac{2}{2}\right)

M=(2,1)M = (2, 1)

Answer: (2,1)(2, 1)

5Problem 5medium

Question:

Point M(4, 7) is the midpoint of segment AB. If A is at (2, 3), find the coordinates of point B.

💡 Show Solution

Step 1: Recall the midpoint formula: M = ((x₁ + x₂)/2, (y₁ + y₂)/2)

Step 2: Set up equations using given information: M(4, 7) and A(2, 3) 4 = (2 + x_B)/2 7 = (3 + y_B)/2

Step 3: Solve for x-coordinate of B: 4 = (2 + x_B)/2 8 = 2 + x_B x_B = 6

Step 4: Solve for y-coordinate of B: 7 = (3 + y_B)/2 14 = 3 + y_B y_B = 11

Step 5: Verify: Midpoint = ((2 + 6)/2, (3 + 11)/2) = (8/2, 14/2) = (4, 7) ✓

Answer: Point B is at (6, 11)

6Problem 6medium

Question:

A circle has center C(2, -1) and passes through point P(5, 3). Find the radius of the circle.

💡 Show Solution

Step 1: Understand the problem: The radius is the distance from center to any point on the circle So radius = distance from C to P

Step 2: Use the distance formula: d = √[(x₂ - x₁)² + (y₂ - y₁)²]

Step 3: Substitute C(2, -1) and P(5, 3): r = √[(5 - 2)² + (3 - (-1))²] r = √[3² + 4²] r = √[9 + 16] r = √25 r = 5

Step 4: Recognize the Pythagorean triple: This is a 3-4-5 right triangle

Answer: The radius is 5 units

7Problem 7hard

Question:

Prove that the triangle with vertices A(0,0)A(0, 0), B(5,0)B(5, 0), and C(5,12)C(5, 12) is a right triangle.

💡 Show Solution

Strategy: Show that the sides satisfy the Pythagorean Theorem.

Find all three side lengths:

ABAB: from (0,0)(0,0) to (5,0)(5,0) AB=(50)2+(00)2=5AB = \sqrt{(5-0)^2 + (0-0)^2} = 5

BCBC: from (5,0)(5,0) to (5,12)(5,12) BC=(55)2+(120)2=12BC = \sqrt{(5-5)^2 + (12-0)^2} = 12

ACAC: from (0,0)(0,0) to (5,12)(5,12) AC=(50)2+(120)2=25+144=169=13AC = \sqrt{(5-0)^2 + (12-0)^2} = \sqrt{25 + 144} = \sqrt{169} = 13

Check Pythagorean Theorem: AB2+BC2=52+122=25+144=169AB^2 + BC^2 = 5^2 + 12^2 = 25 + 144 = 169 AC2=132=169AC^2 = 13^2 = 169

Since AB2+BC2=AC2AB^2 + BC^2 = AC^2, the triangle is a right triangle.

Answer: Yes, it's a right triangle (in fact, a 5-12-13 right triangle)

8Problem 8hard

Question:

Points A(-3, 2), B(5, 8), and C(x, y) form a right triangle with the right angle at B. If C is on the x-axis, find the coordinates of C.

💡 Show Solution

Step 1: Understand the constraints:

  • Right angle at B
  • C is on the x-axis, so y-coordinate = 0: C(x, 0)
  • Need to find x

Step 2: Use perpendicular slopes: If angle at B is 90°, then AB ⊥ BC Slopes must multiply to -1

Step 3: Find slope of AB: m_AB = (8 - 2)/(5 - (-3)) m_AB = 6/8 = 3/4

Step 4: Find slope of BC: m_BC = (0 - 8)/(x - 5) m_BC = -8/(x - 5)

Step 5: Set up perpendicularity condition: m_AB × m_BC = -1 (3/4) × (-8/(x - 5)) = -1

Step 6: Solve for x: (3/4) × (-8)/(x - 5) = -1 -24/(4(x - 5)) = -1 -24 = -4(x - 5) -24 = -4x + 20 -44 = -4x x = 11

Step 7: Verify the perpendicularity: m_AB = 3/4 m_BC = -8/(11 - 5) = -8/6 = -4/3 Product: (3/4) × (-4/3) = -12/12 = -1 ✓

Answer: Point C is at (11, 0)