unit digit of 1+9+9^2+9^3+-------+9^2017 is?
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s=1+9+9^2+9^3+-------+9^2017. ......1
=>9s=9+9^2+9^3+-------+9^2017+9^2018......2
on subtracting 1 &2,
8s=9^2018-1
unit digit will be 0
=>9s=9+9^2+9^3+-------+9^2017+9^2018......2
on subtracting 1 &2,
8s=9^2018-1
unit digit will be 0
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