Question

Find the number of the A.P :
-12, -9, ___, 21. If 1 is added to each terms of this A.P, then find the sum of all its terms of A.P thus obtained.​

Answer 1

a=-12,d=3,tn=21

then,

21=-12+(n-1)3

n=12.....

second condition,,,,,,,

-12+1,-9+1,,,,,,,,,21+1

SN=6(-11+22)

SN=66.

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