Let ax + by + c = 0 be a variable straight line, where a, b and c are first, third and seventh terms of an increasingA.P. Then, the variable straight line always passes through a fixed point which lies on

Let d be the common difference of A.P., thenb = a + 2d and c = a + 6d. Clearly, (b – a) × 3 = c – a⇒ 2a – 3b + c = 0Thus, the straight line ax + by + c = 0 passes throughthe point (2, –3) which also satisfies x2 + y2 = 13.