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Question

The 14th term of an AP is twice its 8th term. If its 6th term is -8, then find the sum of its first 20 terms.

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Solution

Given,

T6=8

a+(61)d=8

a+5d=8

a=85d

T14=2T8

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

a+13d=2a+14d

a=d

85d=d

8=4d

d=2

a=2

s20=2010(2+2+(201)(2))

=10(4+19(2))

=10(438)

=340

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