given the figure
Similar triangles are given by definition when the inner angles of one triangle
are the same as the second triangle
given the angles between parallels theorems
then
then
since the triangle ADB have the same inner angles of BDC
where the angles of the triangle
ADB
∠A
∠D
∠B
are equal to the angles of the triangle
BDC
∠B
∠D
∠C
correspondingly
and they are equals to the angles in the triangle
ABC in the angles
∠A
∠D
∠C
correspondingly