Sir please give me a detailed solution of this question in which we have been given a series of AP 121, 117, 113,…and we have been asked to find its first negative term?
Book – RS Aggarwal, Class 10, Arithmetic Progression, chapter 5A, question no 24
a=121 and
d=117−121
=−4
an=a+(n−1)d
=121+(n−1)(−4)
=121−4n+4
=125−4n
We have find the first term of this A.P.
Therefore an<0
125−4n<0
124<4n
n>125/4
n>31.25
n=32