JEE Advance - Mathematics (1988 - No. 18)
Sum of the first n terms of the series $${1 \over 2} + {3 \over 4} + {7 \over 8} + {{15} \over {16}} + ............$$ is equal to
$${2^n} - n - 1$$
$$1 - {2^{ - n}}$$
$$n + {2^{ - n}} - 1$$
$${2^n} + 1$$
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