How many terms of series must be added to get sum of 55 , 15 , 13 , 11, 9......
Answers
Answered by
0
Answer:
n = 11 or n = 5
Step-by-step explanation:
AP is 15,13,11,9,..
a = 15, d=13−15= −2 , Sn = 55, n = ?
Sn=n[2a+(n−1)d]/2
55 = n[2(15)+(n−1)(−2)]/2
110 = n[30−2n+2]
32n−2n² = 110
16n - n² = 55
n² − 16n + 55 = 0
∴ (n-11)(n-5) = 0
n = 11 or n = 5
Similar questions