11, 7, 3, -1 is an A.P. whose first term is 11 and the common difference is equal to `7-11=4.`

Explanation

Here, `a = –11, d = –7 – (–11) = 4, an = 49`

We have an `= a + (n – 1) d `

So, `49 = –11 + (n – 1) × 4`
i.e., `60 = (n – 1) × 4
`
i.e., `n = 16`

As n is an even number, there will be two middle terms which are `(16/2)th `and `(16/2+1)th`

i.e., the 8th term and 9th term.

`a_8` `= a + 7d = –11 + 7 × 4 = 17 `

`a_9` `= a + 8d = –11 + 8 × 4 = 21 `
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