Which term of the progression is the first negative term? (CBSE 2017)

Given AP is

Whose, first term, a = 20

common difference,

Let nth term of the AP be the first negative term.

Then, we have to find the least value for which

⇒ a_n < 0

And we know, nth term of an AP = a + (n - 1)d

Where a and d are first term and common difference respectively.

⇒ a + ( n - 1 )d < 0

⇒ -3( n - 1 ) < -80

⇒ 3(n - 1) > 80

⇒ 3n - 3 > 80

⇒ 3n = 83

so, n will be least natural number greater than i.e.

i.e., 28th term is the first negative term of the given AP.