Because triangle ABC is an isosceles triangle, AB = AC = X.
Because the circumference of △ABC is 18, 2X+8= 18 =X=5.
So AB=AC=5
2 because △ABC's outer angle ∠c= 120 and inner angle ∠ C = 180- 120 = 60.
∠ B = ∠ A ∠ C = 60, so △ABC is an equilateral triangle, so ∠ A = ∠ B = ∠ C = 60.
3 Because AD is parallel to BC ∠ B = 50, ∠ Bad = 50.
Because AB bisects the angle ∠DAC, ∠ DAC = 100.
So ∠ a = 50.
So ∠ c = 180-50-50 = 80.