Show that a1, a2, ……. an,…… form an AP where an is defined as below:

**(i)** an = 3 + 4n

**(ii)** an = 9 – 5n

Also find the sum of the first 15 terms in each case.

**(i)** an = 3 + 4n

**(ii)** an = 9 – 5n

Also find the sum of the first 15 terms in each case.