if the third term of an A.P is 9and its tenth termis 21, thenthe sum if its first 12th terms is...
third term of an A.P is 9 ---> a+2d = 9
tenth term is 21 ---> a+9d = 21
subtract them:
7d = 12
d = 12/7
in a+2d=9
a+ 24/7 = 9
a = 39/7
sum(12) = (12/2)( 78/7 + 11(12/7)) = 180