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प्रश्न
Find the sum of last 8 terms of the A.P. –12, –10, –8, ……, 58.
उत्तर
First we will reverse the given A.P. as we have to find the sum of last 8 terms of the A.P.
58, …., –8, –10, –12.
Here a = 58, d = –2
`S_n = n/2 [2a + (n - 1) d]`
`=> S_8 = 8/2 [2 xx 58 + (8 -1)(-2)]`
= 4(116 – 14)
= 4 × 102
= 408
Therefore the sum of last 8 terms of the A.P. –12, –10, –8, ……, 58 is 408.
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