Download Lesson 4 - Triangles

NAME ______________________________________________ DATE
9-4
Triangles
____________ PERIOD _____
(Pages 453–457)
A triangle is formed by three line segments that intersect only at their
endpoints. The sum of the measures of the angles of a triangle is 180°.
Classifying
Triangles
You can classify a triangle by its angles.
• An acute triangle has three acute angles.
• A right triangle has one right angle.
• An obtuse triangle has one obtuse angle.
You can classify a triangle by the number of congruent sides.
• An equilateral triangle has 3 congruent sides.
• An isosceles triangle has at least two congruent sides.
• A scalene triangle has no congruent sides.
Try These Together
Use the figure in PRACTICE below to answer these questions.
1. Find m1 if m2 50° and
m 3 55°.
2. Find m1 if m2 65° and
m 3 55°.
HINT: The sum of all angle measures in a triangle is 180°.
Practice
Use the figure at the right to solve each of the following.
2
3. Find m1 if m2 52° and m 3 69°.
4. Find m1 if m2 62° and m 3 44°.
5. Find m1 if m2 71° and m 3 22°.
1
3
6. Find m1 if m2 90° and m 3 30°.
First classify each triangle as acute, right, or obtuse. Then classify
each triangle as scalene, isosceles, or equilateral.
7.
8.
9.
10.
11. Food Samantha likes her grilled cheese sandwiches cut in half
diagonally. Classify the triangles that come from slicing a square
diagonally. Are they acute, right, or obtuse? Are they scalene, isosceles,
or equilateral?
12. Standardized Test Practice Find the measure of A.
A 60°
B 52°
C 50°
D 48°
A
x
B 78
8. obtuse; scalene
9. right; scalene
Glencoe/McGraw-Hill
C
Answers: 1. 75° 2. 60° 3. 59° 4. 74° 5. 87° 6. 60° 7. acute; equilateral
10. acute; isosceles 11. right; isosceles 12. C
©
(x + 2)
75
Glencoe Pre-Algebra