The 14th term of an A.P is twice its 8th term. If its 6th term is -8, then find the sum of
its first 20 terms?
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The 14th term of an A.P is twice its 8th term. If its 6th term is -8, then find the sum of
its first 20 terms?
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Answer:
a14 = 2×a8
a+13d = 2(a+7d)
a+13d = 2a + 14d
a-2a = 14d-13d
-a = 1d
a = -1d
a6 = a+5d
-8 = -d + 5d
-8 = 4d
d = -8/4 = -2
a= -1(-2) = 2
S20 = n/2 (2a+19d)
S20 = 20/2 (2×2 +19(-2))
S20 = 10 (4-38)
S20 = 10(-34)
S20 = -340
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