7. The sum of the first 55 terms of an A. P. is 3300. Find the 28th term.
Sol. S55 = 3300 [Given]
Sn = n/2[2a + (n – 1)d]
∴ S55 = 55/2[2a + (55 – 1) d]
∴ 3300 = 55/2[2a + 54d]
∴ 3300 = 55/2 × 2[a + 27d]
∴ 3300 = 55 [a + 27d]
∴ 3300/55 = a + 27d
∴ a + 27d= 60 ......(i)
Now, tn = a + (n – 1) d
∴ t28 = a + (28 – 1) d
∴ t28 = a + 27d
∴ t28 = 60 [From (i)]
∴ Twenty eighth term of A.P. is 60.