4.7 Triangles and Coordinate Proof

4.7 Triangles and Coordinate Proof Geometry Mrs. Spitz Fall 2004

Objectives: • Place geometric figures in a coordinate plane. • Write a coordinate proof.

Assignment: • pp. 246-249 #1-28 all • Quiz after this section – similar quiz on pg. 250. • Chapter 4 Review – pgs. 252-254 #1-18 • Extra Credit –Algebra Review –pgs. 258-259 #1-66 all – SHOW ALL WORK. • Test next week before break. If you intend to leave early . . . Take the test before you leave. Binders are due that day.

Placing Figures in a Coordinate Plane • So far, you have studied two-column proofs, paragraph proofs, and flow proofs. A COORDINATE PROOF involves placing geometric figures in a coordinate plane. Then you can use the Distance Formula (no, you never get away from using this) and the Midpoint Formula, as well as postulate and theorems to prove statements about figures.

Ex. 1: Placing a Rectangle in a Coordinate Plane • Place a 2-unit by 6-unit rectangle in a coordinate plane. • SOLUTION: Choose a placement that makes finding distance easy (along the origin) as seen to the right.

One vertex is at the origin, and three of the vertices have at least one coordinate that is 0. Ex. 1: Placing a Rectangle in a Coordinate Plane

One side is centered at the origin, and the x-coordinates are opposites. Ex. 1: Placing a Rectangle in a Coordinate Plane

Note: • Once a figure has been placed in a coordinate plane, you can use the Distance Formula or the Midpoint Formula to measure distances or locate points

A right triangle has legs of 5 units and 12 units. Place the triangle in a coordinate plane. Label the coordinates of the vertices and find the length of the hypotenuse. Ex. 2: Using the Distance Formula

One possible placement is shown. Notice that one leg is vertical and the other leg is horizontal, which assures that the legs meet as right angles. Points on the same vertical segment have the same x-coordinate, and points on the same horizontal segment have the same y-coordinate. Ex. 2: Using the Distance Formula

You can use the Distance Formula to find the length of the hypotenuse. d = √(x2 – x1)2 + (y2 – y1)2 = √(12-0)2 + (5-0)2 = √169 = 13 Ex. 2: Using the Distance Formula

In the diagram, ∆MLN ≅ ∆KLN). Find the coordinates of point L. Solution: Because the triangles are congruent, it follows that ML ≅ KL. So, point L must be the midpoint of MK. This means you can use the Midpoint Formula to find the coordinates of point L. Ex. 3 Using the Midpoint Formula

L (x, y) = x1 + x2, y1 +y2 2 2 Midpoint Formula =160+0 , 0+160 2 2 Substitute values = (80, 80) Simplify. Ex. 3 Using the Midpoint Formula

Writing Coordinate Proofs • Once a figure is placed in a coordinate plane, you may be able to prove statements about the figure.

Ex. 4: Writing a Plan for a Coordinate Proof • Write a plan to prove that SQ bisects PSR. • Given: Coordinates of vertices of ∆PQS and ∆RQS. • Prove SQ bisects PSR. • Plan for proof: Use the Distance Formula to find the side lengths of ∆PQS and ∆RQS. Then use the SSS Congruence Postulate to show that ∆PQS ≅ ∆RQS. Finally, use the fact that corresponding parts of congruent triangles are congruent (CPCTC) to conclude that PSQ ≅RSQ, which implies that SQ bisects PSR.

Given: Coordinates of vertices of ∆PQS and ∆RQS. Prove SQ bisects PSR. Ex. 4: Writing a Plan for a Coordinate Proof

NOTE: • The coordinate proof in Example 4 applies to a specific triangle. When you want to prove a statement about a more general set of figures, it is helpful to use variables as coordinates. • For instance, you can use variable coordinates to duplicate the proof in Example 4. Once this is done, you can conclude that SQ bisects PSR for any triangle whose coordinates fit the given pattern.

No coordinates – just variables

Right ∆QBC has leg lengths of h units and k units. You can find the coordinates of points B and C by considering how the triangle is placed in a coordinate plane. Point B is h units horizontally from the origin (0, 0), so its coordinates are (h, 0). Point C is h units horizontally from the origin and k units vertically from the origin, so its coordinates are (h, k). You can use the Distance Formula to find the length of the hypotenuse QC. Ex. 5: Using Variables as Coordinates C (h, k) hypotenuse k units Q (0, 0) B (h, 0) h units

OC = √(x2 – x1)2 + (y2 – y1)2 = √(h-0)2 + (k - 0)2 = √h2 + k2 Ex. 5: Using Variables as Coordinates C (h, k) hypotenuse k units Q (0, 0) B (h, 0) h units

Given: Coordinates of figure OTUV Prove ∆OUT  ∆UVO Coordinate proof: Segments OV and UT have the same length. OV = √(h-0)2 + (0 - 0)2=h UT = √(m+h-m)2 + (k - k)2=h Ex. 5 Writing a Coordinate Proof

Horizontal segments UT and OV each have a slope of 0, which implies they are parallel. Segment OU intersects UT and OV to form congruent alternate interior angles TUO and VOU. Because OU  OU, you can apply the SAS Congruence Postulate to conclude that ∆OUT  ∆UVO. Ex. 5 Writing a Coordinate Proof

4.7 Triangles and Coordinate Proof