UPSC NDA Math Model Paper 1

Concept:
If a, b, c are in A.P then 2b= a+c
Calculations:
Given, the straight line ax+by+c=0 always passes through (1,-2) .
⇒ The point (1,-2) is satisfying the equation of line ax+by+c=0 ⇒a(1)+b(−2)+c=0
⇒2b=a+c
⇒ we can say that 2b=a+c. then a,b,c are in A.P