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प्रश्न
The first and the last terms of an A.P. are 8 and 350 respectively. If its common difference is 9, how many terms are there and what is their sum?
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उत्तर
First term, a = 8
Common difference, d = 9
Let the nth term be the last term.
∴ l = an = 350
⇒ a + (n − 1) d = 350
⇒ 8 + (n − 1) × 9 = 350
⇒ (n − 1) × 9 = 342
`rArr n-1=342/9=38`
`rArrn=38+1=39`
Thus, there are 39 terms in the given A.P.
Sum of 39 trems , `S_39=39/2(a+a_39)`
`=39/2xx(8+350)`
`=39/2xx358`
`=6981`
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