i have doubt in ap question in our yesterday\'s exam how can we assume the ap is a-3d,a-d,a+d,a+3d??

i have doubt in ap question in our yesterday\'s exam how can we assume

1.	i have doubt in ap question in our yesterday\'s exam how can we assume the ap is a-3d,a-d,a+d,a+3d??
Answer» I don\'t have RS Aggarwal It is given in rs agg. Book