The 14term of an Ap twice its 8term if its 6term is 8then find the sum of first 20term
Answers
Answered by
8
a₆ = a+5d
8 =a+5d
Also
a₁₄ = 2a₈
a +13d = 2(a+7d)
a+13d = 2a+14d
a+d =0
a = -d
8=a+5d
8=a-5a
8=-4a
a = -2
d=2
Now
S₂₀ = n/2(2a+(n-1)d)
= 10(-4+28)
=240
8 =a+5d
Also
a₁₄ = 2a₈
a +13d = 2(a+7d)
a+13d = 2a+14d
a+d =0
a = -d
8=a+5d
8=a-5a
8=-4a
a = -2
d=2
Now
S₂₀ = n/2(2a+(n-1)d)
= 10(-4+28)
=240
Similar questions