7 2 Converse of the Pythagorean Theorem Objective

7. 2 Converse of the Pythagorean Theorem Objective: You will use the converse to determine if a triangle is a right triangle.

EXAMPLE 1 Verify right triangles Tell whether the given triangle is a right triangle. a. b. Let c represent the length of the longest side of the triangle. Check to see whether the side lengths satisfy the equation c 2 = a 2 + b 2. a. (3 34)2 =? 92 + 152 9 34 =? 81 + 225 306 = 306 The triangle is a right triangle. b. 262 =? 222 + 142 676 =? 484 + 196 676 = 680 The triangle is not a right triangle.

GUIDED PRACTICE for Example 1 Tell whether a triangle with the given side lengths is a right triangle. 1. 4, 4 3 , 8 ANSWER 2. 10, 11, and 14 ANSWER 3. right triangle 5, 6 and ANSWER not a right triangle 61 right triangle

EXAMPLE 2 Classify triangles Can segments with lengths of 4. 3 feet, 5. 2 feet, and 6. 1 feet form a triangle? If so, would the triangle be acute, right, or obtuse? SOLUTION STEP 1 Use the Triangle Inequality Theorem to check that the segments can make a triangle. 4. 3 + 5. 2 = 9. 5 > 6. 1 4. 3 + 6. 1 = 10. 4 > 5. 2 + 6. 1 = 11. 3 > 4. 3 The side lengths 4. 3 feet, 5. 2 feet, and 6. 1 feet can form a triangle.

EXAMPLE 2 Classify triangles STEP 2 Classify the triangle by comparing the square of the length of the longest side with the sum of squares of the lengths of the shorter sides. c 2 ? a 2 + b 2 6. 12 ? 4. 32 + 5. 22 37. 212 ? 18. 492 + 27. 042 37. 21 < 45. 53 Compare c 2 with a 2 + b 2. Substitute. Simplify. c 2 is less than a 2 + b 2. The side lengths 4. 3 feet, 5. 2 feet, and 6. 1 feet form an acute triangle.

EXAMPLE 3 Use the Converse of the Pythagorean Theorem Catamaran You are part of a crew that is installing the mast on a catamaran. When the mast is fastened properly, it is perpendicular to the trampoline deck. How can you check that the mast is perpendicular using a tape measure? SOLUTION To show a line is perpendicular to a plane you must show that the line is perpendicular to two lines in the plane.

EXAMPLE 3 Use the Converse of the Pythagorean Theorem Think of the mast as a line and the deck as a plane. Use a 3 -4 -5 right triangle and the Converse of the Pythagorean Theorem to show that the mast is perpendicular to different lines on the deck. First place a mark 3 feet up the mast and a mark on the deck 4 feet from the mast. Use the tape measure to check that the distance between the two marks is 5 feet. The mast makes a right angle with the line on the deck. Finally, repeat the procedure to show that the mast is perpendicular to another line on the deck.

GUIDED PRACTICE for Examples 2 and 3 4. Show that segments with lengths 3, 4, and 6 can form a triangle and classify the triangle as acute, right, or obtuse. ANSWER 3 + 4 > 6, 4 + 6 > 3, 6 + 3 > 4, obtuse

GUIDED PRACTICE 5. for Examples 2 and 3 WHAT IF? In Example 3, could you use triangles with side lengths 2, 3, and 4 to verify that you have perpendicular lines? Explain. ANSWER No; in order to verify that you have perpendicular lines, the triangle would have to be a right triangle, and a 2 -3 -4 triangle is not a right triangle.