Measuring and 1-3 Constructing Angles Warm Up Lesson Presentation Lesson Quiz Holt McDougal Geometry Holt Geometry

Warm Up 1. Draw AB and AC, where A, B, and C are noncollinear. 2. Draw opposite rays DE and DF. Solve each equation. 3. 2x + 3 + x – 4 + 3x – 5 = 180 4. 5x + 2 = 8x – 10 C B A Possible answer: E F D 31 4

Objectives Name and classify angles. Measure and construct angles and angle bisectors.

Vocabulary angle right angle vertex obtuse angle interior of an angle straight angle exterior of an angle congruent angles measure angle bisector degree acute angle

A transit is a tool for measuring angles A transit is a tool for measuring angles. It consists of a telescope that swivels horizontally and vertically. Using a transit, a survey or can measure the angle formed by his or her location and two distant points. An angle is a figure formed by two rays, or sides, with a common endpoint called the vertex (plural: vertices). You can name an angle several ways: by its vertex, by a point on each ray and the vertex, or by a number.

The set of all points between the sides of the angle is the interior of an angle. The exterior of an angle is the set of all points outside the angle. Angle Name R, SRT, TRS, or 1 You cannot name an angle just by its vertex if the point is the vertex of more than one angle. In this case, you must use all three points to name the angle, and the middle point is always the vertex.

You can use the Protractor Postulate to help you classify angles by their measure. The measure of an angle is the absolute value of the difference of the real numbers that the rays correspond with on a protractor. If OC corresponds with c and OD corresponds with d, mDOC = |d – c| or |c – d|.

Example 2: Measuring and Classifying Angles Find the measure of each angle. Then classify each as acute, right, or obtuse. A. WXV mWXV = 30° WXV is acute. B. ZXW mZXW = |130° - 30°| = 100° ZXW = is obtuse.

Congruent angles are angles that have the same measure Congruent angles are angles that have the same measure. In the diagram, mABC = mDEF, so you can write ABC  DEF. This is read as “angle ABC is congruent to angle DEF.” Arc marks are used to show that the two angles are congruent. The Angle Addition Postulate is very similar to the Segment Addition Postulate that you learned in the previous lesson.

Example 3: Using the Angle Addition Postulate mDEG = 115°, and mDEF = 48°. Find mFEG mDEG = mDEF + mFEG  Add. Post. 115 = 48 + mFEG Substitute the given values. –48° –48° Subtract 48 from both sides. 67 = mFEG Simplify.

An angle bisector is a ray that divides an angle into two congruent angles. JK bisects LJM; thus LJK  KJM.

Example 4: Finding the Measure of an Angle KM bisects JKL, mJKM = (4x + 6)°, and mMKL = (7x – 12)°. Find mJKM.

Example 4 Continued Step 1 Find x. mJKM = mMKL Def. of  bisector (4x + 6)° = (7x – 12)° Substitute the given values. +12 +12 Add 12 to both sides. 4x + 18 = 7x Simplify. –4x –4x Subtract 4x from both sides. 18 = 3x Divide both sides by 3. 6 = x Simplify.

Example 4 Continued Step 2 Find mJKM. mJKM = 4x + 6 = 4(6) + 6 Substitute 6 for x. = 30 Simplify.