Find the number of terms of the A.P. If 1 is added to each term of this A.P., then find the sum of all terms of the A.P. thus obtained.
The given A.P. is -12, -9, -6,…21
Here, a = -12, d = -9 – (-12) = 3
Let the number of terms be n
Tn = a + (n – 1) d
21 = -12 + (n – 1) (3)
21 = -15 + 3n
36 = 3n
n = 12
If 1 is added to each term than A.P. is
-11, -8, -5…20
Here, a = -11, d = 3
S12= [2(-11) + (12 -1) * 3]
= 6 (-22 + 33)
= 66