Question

find the number of terms of AP -12,-9,-6...21, if 1 added to each term of this AP then find the sum of all terms of the AP

Answer 1

Answer:

(-12,-9,-6,.........21)+1= -11,-8,-5.........22

à= -11

d=a2-a1

d= -8-(-11)

d=-8+11

d=3

an=22

An=a+(n-1)d

22= -11+(n-1)3

22= -11+(3n-3)

22=-11+3n-3

22+11+3=3n

36=3n

36/3=n

12=n

Sn=n/2(a+an)

Sn=12/2(-11+22)

Sn=6(11)

Sn=66