The 14th term of an AP is twice its 8th term. IF the 6th term is -8, then find the sum of its first 20 terms. pls help guys!!
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Answered by
7
Answer:
-340
Step-by-step explanation:
A.T.Q.
1ST CONDITION
A14 = 2(A8)
A+13D = 2(A+7D)
A+13D = 2A+14D
2A + 14D - A - 13D = 0
A +D = 0 ----- 1st equation
2ND CONDITION
A6 = -8
A+5D = -8 ----- 2nd equation
Subtract equation 1 and 2
A+ D = 0
A+5D= -8
- - = +
-4D = 8
D = -2
PUT IN 1
A -2 = 0
A = 2
SUM OF 1ST 20 TERMS IS
S20 = 20/2 [2×2+(20-1) × -2]
= 10 [ 4 - 38 ]
= 10 × -34
= -340
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