Question

a=3 d=3 n=12 find sn

Answer 1

Sn=n/2{2a+(n-1)d}

6{6+33)

39×6

234

Answer 2

Sn=n/2(a+(n-1)d)

substitute the values of a,d and n

Sn=12/2(3+(12-1)3)

   = 6(3+(11)3)

   = 6(3+33)

   = 6(36)

  = 216.

therefore , Sn=216