20ai) According to parallelogram law, vector OA=vector CB. Because vector BC is opposite of vector CB, change the direction by multiplying by -1

20aii) Triangle law of vector addition

20bii) In future, note that in a trapezium, when divided into two from its diagonals forms two triangles. Note area of trapezium as 1/2 x sum of two parallel sides divided by 2 x height. The height will always be the same.