Here, P = 1000, r = 8%
Interest at the end of first year.
Interest at the end of second year
Interest at the end of third year
So, we have 80, 160, 240, ..............
Here, a = 80, d = 80
Therefore, interest at the end of 30 yrs. i.e. a30 is,
a30 = a + (30 –1) d = 80 + 29 x 80
= 80 + 2320 = 2400
So the interest at the end of 30 years is Rs. 2400.
The first and the last term of an A .P. are 4 and 81 respectively. If the common difference is 7, how many terms are there in the A .P. and what is their sum?