Question

If the nth term of an AP is 7 - 3n, find the
sum of twenty five terms. ​

Answer 1

Answer:

Let first term of AP be, a

Common difference be d

According to question,

nth term is  =>   7−3n=a+(n−1)d

=>  a+nd−d=7−3n

By comparing coefficents, we get

a=4 &  d=−3

So, sum first 25 terms,

2

n

​  

[2a+(n−1)d]

=>    

2

25

​  

[2×4+(25−1)(−3)]

=>    

2

25

​  

[8−72]

=>  −400