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.
Answers
Answered by
18
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
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