Question

21. The first term of an AP is 7 and the sum of its first four terms is half the sum of
its next four term. Find the sum of its first 30 terms.

Answer 1

Answer:

1428

Step-by-step explanation:

S4 = ½(S8-S4)

2[14+ 3d] = ½[ 4(14 + 7d) - 2(14+3d)]

2 [28+6d] = 56 + 28 d -28 -6d

56+12d =56 -28 +22d

28 = 10 d

d = 2.8

S30 = 15[ 14+29(2.8)]

= 15 [ 14 + 81.2]

= 15(95.2)

= 1428