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Question
Find the sum of 25 terms of the A.P. 8 + 5 + 2 + .... .
Sum
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Solution
a = 8
d = 5 − 8
= −3
n = 25
`S_n = n/2 [2a + (n - 1)d]`
`S_25 = 25/2 [2(8) + (25 - 1)(-3)]`
= `25/2[16 + 24(−3)]`
= `25/2[16 + (-72)]`
= `25/2 [-56]`
= `25 xx (-56)/2`
= 25 × (−28)
= − 700
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