Concept Let us consider sequence a1,a2,a3... an is an A.P. Common difference " d"=a2−a1=a3−a2=....=an−an−1 Calculation: Given A,B,C are in AP. So we can write 2B=A+C A−2B+C=0 . . . (i) Given equation of straight line is Ax+2By+C=0...(ii) Comparing (i) and (ii) x=1,y=−1 So (1,−1) lies on the line.