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The Pythagorean Theorem Objectives Use the Pythagorean Theorem and its converse to solve problems. Use Pythagorean inequalities to classify triangles. Holt McDougal Geometry The Pythagorean Theorem Vocabulary Pythagorean triple Holt McDougal Geometry The Pythagorean Theorem The Pythagorean Theorem is probably the most famous mathematical relationship. As you learned in Lesson 1-6, it states that in a right triangle, the sum of the squares of the lengths of the legs equals the square of the length of the hypotenuse. a2 + b2 = c2 Holt McDougal Geometry The Pythagorean Theorem Example 1A: Using the Pythagorean Theorem Find the value of x. Give your answer in simplest radical form. a2 + b2 = c2 Pythagorean Theorem 22 + 62 = x2 Substitute 2 for a, 6 for b, and x for c. 40 = x2 Simplify. Find the positive square root. Simplify the radical. Holt McDougal Geometry The Pythagorean Theorem Example 1B: Using the Pythagorean Theorem Find the value of x. Give your answer in simplest radical form. a2 + b2 = c2 Pythagorean Theorem (x – 2)2 + 42 = x2 Substitute x – 2 for a, 4 for b, and x for c. x2 – 4x + 4 + 16 = x2 Multiply. –4x + 20 = 0 Combine like terms. 20 = 4x Add 4x to both sides. 5=x Holt McDougal Geometry Divide both sides by 4. The Pythagorean Theorem Check It Out! Example 2 What if...? According to the recommended safety ratio of 4:1, how high will a 30-foot ladder reach when placed against a wall? Round to the nearest inch. Let x be the distance in feet from the foot of the ladder to the base of the wall. Then 4x is the distance in feet from the top of the ladder to the base of the wall. Holt McDougal Geometry The Pythagorean Theorem Check It Out! Example 2 Continued a2 + b2 = c2 (4x)2 + x2 = 302 17x2 = 900 Pythagorean Theorem Substitute 4x for a, x for b, and 30 for c. Multiply and combine like terms. Since 4x is the distance in feet from the top of the ladder to the base of the wall, 4(7.28) 29 ft 1 in. Holt McDougal Geometry The Pythagorean Theorem A set of three nonzero whole numbers a, b, and c such that a2 + b2 = c2 is called a Pythagorean triple. Holt McDougal Geometry The Pythagorean Theorem The converse of the Pythagorean Theorem gives you a way to tell if a triangle is a right triangle when you know the side lengths. Holt McDougal Geometry The Pythagorean Theorem You can also use side lengths to classify a triangle as acute or obtuse. B c A Holt McDougal Geometry a b C The Pythagorean Theorem To understand why the Pythagorean inequalities are true, consider ∆ABC. Holt McDougal Geometry The Pythagorean Theorem Remember! By the Triangle Inequality Theorem, the sum of any two side lengths of a triangle is greater than the third side length. Holt McDougal Geometry The Pythagorean Theorem Example 4A: Classifying Triangles Tell if the measures can be the side lengths of a triangle. If so, classify the triangle as acute, obtuse, or right. 5, 7, 10 Step 1 Determine if the measures form a triangle. By the Triangle Inequality Theorem, 5, 7, and 10 can be the side lengths of a triangle. Holt McDougal Geometry The Pythagorean Theorem Example 4A Continued Step 2 Classify the triangle. c2 102 ? = a2 + b2 ? = 52 + 72 ? Compare c2 to a2 + b2. Substitute the longest side for c. 100 = 25 + 49 Multiply. 100 > 74 Add and compare. Since c2 > a2 + b2, the triangle is obtuse. Holt McDougal Geometry The Pythagorean Theorem Example 4B: Classifying Triangles Tell if the measures can be the side lengths of a triangle. If so, classify the triangle as acute, obtuse, or right. 5, 8, 17 Step 1 Determine if the measures form a triangle. Since 5 + 8 = 13 and 13 > 17, these cannot be the side lengths of a triangle. Holt McDougal Geometry The Pythagorean Theorem Check It Out! Example 4a Tell if the measures can be the side lengths of a triangle. If so, classify the triangle as acute, obtuse, or right. 7, 12, 16 Step 1 Determine if the measures form a triangle. By the Triangle Inequality Theorem, 7, 12, and 16 can be the side lengths of a triangle. Holt McDougal Geometry The Pythagorean Theorem Check It Out! Example 4a Continued Step 2 Classify the triangle. c2 162 ? = a2 + b2 ? = 122 + 72 ? Compare c2 to a2 + b2. Substitute the longest side for c. 256 = 144 + 49 Multiply. 256 > 193 Add and compare. Since c2 > a2 + b2, the triangle is obtuse. Holt McDougal Geometry The Pythagorean Theorem Check It Out! Example 4b Tell if the measures can be the side lengths of a triangle. If so, classify the triangle as acute, obtuse, or right. 11, 18, 34 Step 1 Determine if the measures form a triangle. Since 11 + 18 = 29 and 29 > 34, these cannot be the sides of a triangle. Holt McDougal Geometry The Pythagorean Theorem Check It Out! Example 4c Tell if the measures can be the side lengths of a triangle. If so, classify the triangle as acute, obtuse, or right. 3.8, 4.1, 5.2 Step 1 Determine if the measures form a triangle. By the Triangle Inequality Theorem, 3.8, 4.1, and 5.2 can be the side lengths of a triangle. Holt McDougal Geometry The Pythagorean Theorem Check It Out! Example 4c Continued Step 2 Classify the triangle. c2 5.22 ? = a2 + b2 ? = 3.82 + 4.12 Compare c2 to a2 + b2. Substitute the longest side for c. ? 27.04 = 14.44 + 16.81 Multiply. 27.04 < 31.25 Add and compare. Since c2 < a2 + b2, the triangle is acute. Holt McDougal Geometry