Find Sn for d = 7, n = 20, and An = 136

Answer 1

I will assume An is supposed to mean term(n) ??

term(n) = a + (n-1)d
136 = a + 19(7)
a = 3

so sum(n) or your Sn
= (n/2)(2a + (n-1)d )
= (n/2)(6 + 7(n-1) )
= (n/2)( 7n -1 )
or
= (7n^2 - n)/2 or some other variation of that