∴ Twenty eighth term of A.P. is 60.
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The sum of the first 55 terms of an A. P. is 3300. Find the 28th term.
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.