Question

How many terms of the arthmetic sequence 6,10,14 should be added to get 720

Answer 1

Answer:

Let n be the number of terms

Then 720 = (n/2)((2×6 + (n - 1)×4)

Here 1st term is 6 and common difference is 4

720 = n(12+4n-4)/2

720=n(4n+8)/2

720=n(2n+4)

n(n+2)=360

n^2 + 2n -360 = 0

n^2 + 20n - 18n -360=0

(n+20)(n-18)=0

This gives n=-20 and 18

since minus is not possible, hence ans is 18