If all the vertices of a trapezium lie on a circle, then it is a cyclic trapezium.
Opposite angles of a cyclic trapezium are supplementary.
Adjacent angles of a cyclic trapezium are supplementary.
A cyclic trapezium is an isosceles trapezium in which non-parallel sides are equal.
let x be the small angle
since opposite angles are supplementary,
then x+3x=180
4x=180
x=45 degrees.