Q6 of 85 Page 104

If the first term and the last term of a finite A.P. are 5 and 95 respectively and d = 5, find n and Sn.

Formula used.


an = a + (n–1)d


Sn = [2a + (n–1)d]


⇒ d = 5


⇒ a = 5


⇒ l = 95


an = a + (n–1)d


95 = 5 + (n–1)5


95 = 5–5 + 5n


n = = 19


Sn = [a + l]


= [5 + 95]


= 19×50


= 950


∴ Number of terms is 19 and Sum of A.P is 950


More from this chapter

All 85 →
4

How many terms of A.P., 2, 7, 12, 17, ... add up to 990?

 5

The first term of finite A.P. is 5, the last term is 45 and the sum is 500. Find the number of terms.

 7

The sum of first n terms of an A.P. is 5n — 2n2. Find the A.P. i.e. a and d.

 8

Find the sum of all three–digit numbers divisible by 3.