Question

In an ap first term is-5 and last is 45 . Sum of all mo is 120 then how many no are there what is the common difference?

Answer 1

Let d be the c.d and n be the total no of terms
a=-5
Last term=45
a+(n-1)d=45
(n-1)d=50---------------------------eq(1)+
Sn=120
n/2(2a+(n-1)d)=120
n(-10+(n-1)d)=240
From eq(1)
n{-10+(n-1)50/(n-1)}=240
n(-10+50)=240
n=240/40
n=6
d = 10