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Cogito

Geometry · Chapter 3 · Lesson 3

Practice Worksheet

Isosceles and Equilateral Triangles

Equal sides force equal angles.

11 problems · about 22 minutes · G-CO.C.10

NameDate

Figure — use these to answer the problems

Two equal base angles. The apex takes whatever is left of 180°.
030609012015018070° + 70° = 140°
First base angle70
Second base angle70
Figure 1

Warm Up

Straightforward practice. Get the method working first.

4 problems

  1. Base angles 55°. Apex, in degrees?

    Answer

  2. Base angles 50°. Apex, in degrees?

    Answer

  3. Apex 40°. One base angle, in degrees?

    Answer

  4. Two angles are 45° and 45°. Is it isosceles? 1 for yes, 0 for no.

    Answer

Build It Up

The same ideas with more to keep track of.

3 problems

  1. Apex 90°, isosceles. One base angle, in degrees?

    Answer

  2. How many equal sides does an equilateral triangle have?

    Answer

  3. Sort each triangle by how many equal sides it has.

    Write each item under the heading it belongs to: Equilateral · Isosceles · Scalene

    Three equal

    Exactly two equal

    No equal sides

Stretch Yourself

Mixed problems. Work out what kind of question it is before you start.

4 problems

  1. Base angles 30°. Apex, in degrees?

    Answer

  2. Is every equilateral triangle also isosceles? 1 for yes, 0 for no.

    Answer

  3. The Apex

    Base angles 70°. What is the apex, in degrees?

    Answer

  4. Equilateral

    One angle of an equilateral triangle, in degrees?

    Answer