a(n) = a + (n-1)d,
s(n) = na + n(n-1)d/2.
s(2n-1) = (2n-1)a + (2n-1)(n-1)d = (2n-1)[a + (n-1)d] = (2n-1)a(n),
s(11)/s(5) = 11a(6)/[5a(3)] = (11/5)[a(6)/a(3)] = (11/5)(10/7) = 22/7