Question

Find the sum of AP 1,3,5,7..... to 12 terms

Answer 1

AP=1,3,5,7....
a=1
d=3-1
  =2
n=12
Sn=n/2{2a+(n-1)d}
S12=12/2{2*1+(12-1)2}
       =6{2+22}
        =6{24}
        =144

Answer 2

a=1 
d=2
n=12

Sn=n/2(2a+(n-1)d)
Sn=12/2(2*1+12-1(2)
    =6(2+22)
    = 6(24)
    =144