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