Objective: how to find the perimeter of a trapezoid

Which of the following solutions is more sound in terms of naive physics reasoning?

OPTIONS:
- Perimeter = add lengths of all sides    P = a + b1 + b2 + c x b1
- Perimeter = add lengths of all sides    P = a + b1 + b2 + c
Perimeter = add lengths of all sides    P = a + b1 + b2 + c