find the sum of first 21 teems of an AP whose 2nd term is 8 and 4th term is 4
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kanchi804:
sorry but answer is -210
i did the same as you
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Answer: -210
Step-by-step explanation:
2nd term === A2 = 8
4th term === A4 = 4
Common difference, d = (A4 - A2)/4-2 = 4-8/2 = -2
2nd term == a + d = 8
a - 2 = 8
a = 10
To find Sum of 21 terms:
Sn = n/2 *[2a + (n-1)*d]
= 21/2 * [2*10 + 20*-2]
= 21/2 * [20-40]
= 21/2 * -20 = 21*-10 = -210
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