Question

Which term of ap: 121,117,113,....... is its first negetive term

Answer 1

here ;121, 117, 113, ....

             a = 121, d = 117 – 121 = – 4

             an=a+(n−1)d

             = 121 + (n – 1) × – 4

             = 121 – 4n + 4 = 125 – 4n

             For the first negative term  

             an<0⇒125−4n<0  

             ⇒ 125 < 4n  

             ⇒ 1254<n

             ⇒ 3114<n

             n is an integer and n>3114

             ⇒ The first negative term is 32nd term