In a circle, the peripheral angle is half of the corresponding central angle.

Chords with the same radius have a central angle of 60 and a peripheral angle of 30 (the two ends of the chord are connected with the center to form a triangle). Similarly, a chord with a radius twice the root number corresponds to a central angle of 90 and a circumferential angle of 45.

The angle corresponding to the whole circumference can be 360, so the corresponding circumferential angle can be considered as 180 (the sum of circumferential angles corresponding to chords AB, BC, CD and AD is180).

Solution: Connect the AD. AC and BD meet at point e.

Angle ABC+ angle B = (180-45-30).

In △ABE, α= 180- angle ABC- angle B = 75.