FOURTH LESSON. DEFINITIONS. (Illustrate on black board.) 1. A triangle is a figure of three sides and three angles. 2. If the sides be equal, the triangle is termed equilateral. 3. If two sides only are equal, it is termed isosceles. 4. If all are unequal, it is called scalene. 5. A right-angled triangle contains a right angle; the hypothenuse is the line connecting its extremities. 6. An obtuse-angled triangle contains an obtuse angle. 7. An acute-angled triangle contains an acute angle. 8. The vertex of a triangle is its highest point. EXERCISES. Point to a triangle in this room. Draw a 6 inch square. Draw (Fig. 23.) An Divide it into triangles. (Fig. 22.) State their kind. an equilateral triangle with 8 inch sides. isosceles triangle any size. (Fig. 24.) A scalene triangle. (Fig. 25.) Draw two equilateral triangles, one within the other; the outer one with 12 inch sides, the inner one with 6 inch sides. (Fig. 26.) Draw a square 8 inches wide. On its base construct a triangle. (Fig. 27.) State what kind of triangle is formed. Draw an acute-angled triangle. (Fig. 28.) An obtuse-angled triangle. (Fig. 29.) Copy Exercise 30. Copy the Roman figure, Exercise 31. Explain each angle and triangle. See Appendix, D. |