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units digit

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units digit

by finance » Sat Jul 02, 2011 12:50 am
If x & y are both positive integers, and x>y, what's the units digit of (10^x-9^y)^2=?

A) 1
B) 3
C) 5
D) 8
E) 9

I thought it should be 9, but that's not the answer.
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by Frankenstein » Sat Jul 02, 2011 1:24 am
Hi,
10^x ends with 0
9^y end with either 9 or 1
So, 10^x-9^y ends with 1 or 9
Any number ending with 1 or 9 when squared ends with 1.

Hence, A
Cheers!

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Master | Next Rank: 500 Posts
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by finance » Sat Jul 02, 2011 1:30 am
oh, I understood where I was careless now. Thank you Frankenstein!
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