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The first and last term of an A.P. are a...

The first and last term of an A.P. are `a` and `l` respectively. If `S` is the sum of all the terms of the A.P. and the common difference is given by `(l^2-a^2)/(k-(l+a))` , then `k=` (a)`S `(b) `2S` (c) `3S` (d) none of these

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