1, because DE||BC, according to two straight lines parallel, so congruent angles are equal. Angle ADE= angle ABC. Angle AED= Angle ACB = 60°, so the triangle ADE is an equilateral triangle.

Let's assume that the triangle ABC is a right triangle, let's assume that AC is the hypotenuse, and the right-angled side 2*AB=AC, according to the sine value sin30= 1/2.

In a right triangle, ABC. sinACB=AB/AC= 1/2。

So it is thirty degrees.