Question

The sum of first 7 terms of an A.P. is 21 and the sum of its next 7 terms is 161. Find the 28th term of
the A.P. ​

Answer 1

Answer:

Step-by-step explanation:

S7 = 21

(7/2){2*a + (7-1)*d} = 21

2*a + 6*d = 6

a + 3*d = 3 ------(i)

Sum of next 7 terms is 161

S14 - S7 = 161

(14/2){2*a + 13*d} - 21 = 161

7(2*a + 13*d) = 224

a + (13/2)*d = 16 ---------(ii)

subtracting i from ii

(13/2-3) *d = 16 - 3 = 13

(13-6/)*d = 13

d = 13/7

So, from (ii)

a + 13/2 * 13/7 = 16

a = 16 - (26 + 91)/14 =

Similarly, find the a28 th term by

a28 = a + 27d

Thanks