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Math lesson plan · 9th grade (Year 10 in England) · 60 minutes

Write the equation of a line from two points

Algebra

CCSS HSF.LE.A.2CCSS HSA.CED.A.2England: GCSE Foundation AlgebraEngland: GCSE Higher Algebra

Objective and success criteria

Students will: Write the equation of a line from two points.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Use point-slope form y − y₁ = m(x − x₁)

Leads to: Write the equation of a parallel line through a point

Materials: Balance scales or a balance drawing, Algebra tiles, Function machine cards, Mini whiteboards and pens, Printed student sheet (below).

Warm-up · 6 min

  • Quick retrieval from earlier learning (Use point-slope form y − y₁ = m(x − x₁)). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Which equation is the line through (−2, 2) with slope 3? (y − 2 = −3(x + 2) / y − 2 = 3(x + 2) / x + 2 = 3(y − 2) / y + 2 = 3(x − 2)) → y − 2 = 3(x + 2)Prompt: Point-slope form: y − y₁ = m(x − x₁).
  2. Write y − 2 = 5(x + 6) in the form y = mx + b. Type the right-hand side. → 5x+32Prompt: Expand the bracket first.
  3. Arjun's phone plan costs 5 more for each extra gigabyte of data. A month with 7 GB costs 50. Using cost = m × GB + b, find b, the fixed monthly charge. → 15Prompt: The slope m is the cost per gigabyte.

Hook · 6 min

  • Show a "think of a number" puzzle and ask the class to work out the starting number.
  • Share the objective: Write the equation of a line from two points. Read the success criteria together.

I do: model it · 12 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: Find the equation of the line through (−3, −5) and (3, −1). Type the right-hand side of y = … (fractions are fine).
  • Step 1: m = 4/6 = 2/3
  • Step 2: y = (2/3)x − 3
  • Answer: (2/3)x-3
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 12 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. Find the equation of the line through (−2, 6) and (−4, 7). Type the right-hand side of y = … (fractions are fine). → -(1/2)x+5Prompt: The slope will be a fraction this time.
  2. A line passes through (1, −1) and (3, −9). The point (−2, k) is also on the line. Find k. → 11Prompt: Find the equation of the line first.

You do: independent practice · 17 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Find the equation of the line through (−2, −2) and (0, −3). Type the right-hand side of y = … (fractions are fine). → -(1/2)x-3Prompt: The slope will be a fraction this time.
  2. A line passes through (4, −16) and (−5, 20). The point (0, k) is also on the line. Find k. → 0Prompt: Find the equation of the line first.

Watch out for

  • Doing an operation to one side of an equation but not the other.
  • Reading 3x as 3 + x, or thinking x always stands for one particular number.

Support and stretch

Support

  • Review first: Use point-slope form y − y₁ = m(x − x₁).
  • Use concrete materials: balance scales or a balance drawing.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Move on to: Write the equation of a parallel line through a point.
  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-9.

Free lesson plan from eurekami.com/lesson-plan-builder

eurekami

Write the equation of a line from two points

Name

Date

Score/ 8

  1. Question 1

    Find the equation of the line through (4, −15) and (−2, 9). Type y = … (just the right-hand side).

    xy−6−4−2246−15150(4, −15)(−2, 9)

    Answer

  2. Question 2

    Find the equation of the line through (−2, −4) and (4, −1). Type the right-hand side of y = … (fractions are fine).

    Answer

  3. Question 3

    A line passes through (−1, 7) and (2, −2). The point (5, k) is also on the line. Find k.

    Answer

  4. Question 4

    Find the equation of the line through (2, −5) and (3, −6). Type y = … (just the right-hand side).

    xy−6−4−2246−15150(2, −5)(3, −6)

    Answer

  5. Question 5

    Find the equation of the line through (−3, 3) and (3, 1). Type the right-hand side of y = … (fractions are fine).

    Answer

  6. Question 6

    A line passes through (−3, 16) and (−2, 12). The point (−4, k) is also on the line. Find k.

    Answer

  7. Question 7

    Find the equation of the line through (−3, −13) and (−4, −17). Type y = … (just the right-hand side).

    xy−6−4−2246−15150(−3, −13)(−4, −17)

    Answer

  8. Question 8

    Find the equation of the line through (0, 0) and (−3, 2). Type the right-hand side of y = … (fractions are fine).

    Answer

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Answer key: Write the equation of a line from two points

  1. 1. -4x+1

  2. 2. (1/2)x-3

  3. 3. -11

  4. 4. -x-3

  5. 5. -(1/3)x+2

  6. 6. 20

  7. 7. 4x-1

  8. 8. -(2/3)x