Q. 234.3( 4 Votes )

How many terms of the A.P. -6,-,-5 ... are needed to get the sum - 25?

Answer :


Here, a = -6,



and Sn = -25


We know that,







-100 = n[-25 + n]


n2 – 25n + 100 = 0


n2 – 20n – 5n + 100 = 0


n(n – 20) - 5(n – 20) = 0


(n – 20)(n – 5) = 0


n – 5 = 0 or n – 20 = 0


n = 5 or n = 20


So, n = 5 or 20


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