Proof: Because the angle BDA== 1 10, the angle B=40 degrees.
So the angle BAD=30 degrees
Because AB=AC and angle B=40 degrees.
So the angle BAC= 100 degrees.
So the angle DAE=70 degrees
In the triangle ADE, because the angle DAE=70 degrees, the angle ADE=40 degrees.
AED angle =70 degrees.
So AD=ED
So triangle ADE is an isosceles triangle.