11.
The 9th term of an A.P. is 449 and 449th term is 9. The term which is equal to zero is

12.
If Sn denote the sum of n terms of an A.P. with first term a and common difference d such that $$\frac{{{S_x}}}{{{S_{kx}}}}$$ is independent of x, then

13.
The sum of n terms of two A.P.’s are in the ratio 5n + 4 : 9n + 6. Then, the ratio of their 18th term is

14.
The sum of first 20 odd natural numbers is

15.
The next term of the A.P., $$\sqrt 7 ,$$ $$\sqrt {28} ,$$ $$\sqrt {63} ,$$ . . . . . .

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