Find the sum of first 21 terms of the ap whose 2nd term is 8 and 4 term is 4
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Answered by
10
Answer:
-210
Step-by-step explanation:
2nd term = 8 = a+(2-1)d = 8
a + d = 8 ---(1)
4th term = 4 = a+(4-1)d = 4
a+3d = 4 ---(2)
(2)-(1) = 2d = -4 = d = -2
a-2 = 8
a = 10
Sn = n/2[2a+(n-1)d]
S21 = 21/2[2×10 + 20(-2)]
= 21[10-20]
= 21(-10)
= -210
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1
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