I didn't undstood how this1/4 * in Area AOD= 2[{1/4*√3/2 1/2 sin-¹(-1/

This question was asked in Ex 8.2, 2

i didn't undstood how this1/4 * in Area AOD= 2[{1/4*√3/2 1/2 sin-¹(-1/2)} {0 1/2 sin-¹(-1)}]
come instead it should be 1/2...