UPSC NDA I 2021 Mathematics Solved Paper

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.