If the ninety term of an A.P is 30, find the Sum of its first seventeen terms

Answer 1

I will assume you meant the ninetieth term.

If a and d are the first term and common difference respectively, then
term(90) = 30
a + 89d = 30
a = 30-89d

sum(17) = (17/2)(2a + 16d)
= (17/2)(2(30-89d) + 16d)
= (17/2)(60 - 162d)