Question

How many terms of parallel series 2, 7, 12, 17 total sum 990?​

Answer 1

Answer:

Let n terms of AP add upto 990

Sum of n terms of AP

Sn = n/2(2a + (n - 1)d)

Given a = 2, d = 7-2 = 5

Then,

990 = n/2(2*2 + (n - 1)5)

990 = n/2(4 + 5n - 5)

990*2 = n(5n - 1)

5n^2 - n - 1980 = 0

5n^2 - 100n + 99n - 1980 = 0

5n(n - 20) + 99(n - 20) = 0

(5n +99)(n - 20) = 0

n = - 99/5, 20

Therefore sum of 20 terms of given AP is 990