1.

The sum of the first 9 terms of an AP is 81 and that of its first 20 terms is 400. Find the first term and the common difference of the AP

Answer» Correct Answer - a = 1, d = 2
`(9)/(2)(2a+ 8d) = 81 rArr a +4d = 9 " "…(i)`
`(20)/(2)(2a+19d) = 400 rArr 2a +19d = 40 ." "..(ii)`


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