HG 5
D11. Given lines AB and CD with E € AB and CD such that A • E • B and C • E •D. Then – AEC and – BED are called vertical angles. (– AED and – CEB are also vertical angles).
T2. Vertical angles are congruent.
D12. A triangle with exactly two congruent sides is called an isosceles triangle.
T3. Base angles of an isosceles triangle are congruent.