Required area = area of the region ACBPA = (area of the region OACBO) – (area of the region OADBO)
Now, area of the region OACBO = area under the circle x2 + y2 = 1 between x = 0 and x = 1
Area of the region OADBO = area under the line x + y = 1 between x = 0 and x = 1