Midpoint Theorem

In trapezium $ABCD$ , $AB||CD$ and $AB=2CD$ . $M,N$ are the midpoints of $AC$ and $BD$ respectively. Let the perimeter of $ABCD$ be $a$ and the perimeter of quadrilateral $CDMN$ be $b$ . Find $a\div b$ .

Give your answer to 3 decimal places.