Lesson 14

Coordinate Proof

Lesson Narrative

In this lesson, students use coordinates to make conjectures and prove simple geometric theorems algebraically. They begin with some informal reasoning in a “Which One Doesn’t Belong” prompt. In the next activity, students use slopes to classify a quadrilateral. Then, they use inductive reasoning to observe a pattern and make a conjecture which will be generalized in a subsequent unit. Students have an opportunity to attend to precision in mathematical language (MP6) as they write and refine their conjectures. At the end of the lesson, students critique each other’s reasoning (MP3) about properties of the quadrilaterals from the warm-up.

One of the activities in this lesson works best when each student has access to devices that can run the Desmos applet, because students will benefit from seeing the relationship in a dynamic way.

Learning Goals

Teacher Facing

  • Prove simple geometric theorems algebraically using coordinates.

Student Facing

  • Let’s use coordinates to prove theorems and to compute perimeter and area.

Required Materials

Required Preparation

Devices and index cards are required for the digital version of the Circular Logic task. 

Learning Targets

Student Facing

  • I can use coordinates of figures to prove geometric theorems.

CCSS Standards

Glossary Entries

  • opposite

    Two numbers are opposites of each other if they are the same distance from 0 on the number line, but on opposite sides.

    The opposite of 3 is -3 and the opposite of -5 is 5.

  • point-slope form

    The form of an equation for a line with slope \(m\) through the point \((h,k)\). Point-slope form is usually written as \(y-k = m(x-h)\). It can also be written as \(y = k + m(x-h)\).

    A line with point h comma k on an x y axis.
  • reciprocal

    If \(p\) is a rational number that is not zero, then the reciprocal of \(p\) is the number \(\frac{1}{p}\).