the 5th term of an ap is twice its 8th term. if its 6th term is -3 then find the sum of its first 20 term?
sarwarkhan68:
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Step-by-step explanation:
5th term = 1/2 ×8th term
a + 4d. = 1/2 ×(a+7d)
2a +8d = a+ 7d
2a-a = 7d-8d
a = d. eq.(1)
6th term = -3
a +5d = -3
from equation 1
a + 5a = -3
6a = -3
a = -1/2 = d
sum of its first 20 term = n/2[2a + (n-1)d]
20/2[2×(-1/2) + (20-1)(-1/2)]
10[(-1)+(-19/2)]
10[-10.5]
-105
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