1. Given a trapezoid (not a parallelogram) $A B C D$. It is known that triangles $A B D$ and $C B D$ are isosceles, and angle $A$ is 40 degrees. Find all possible values of angle $C$ and list them in ascending order separated by a semicolon.
Answer: $55 ; 70 ; 100 \mid 55,70,100$