Math Is Fun Forum

niharika_kumar

Member

From: Numeraland

Registered: 2013-02-12

Posts: 1,062

exponents

units digit in the number (12357)^655

please tell me the solution!!

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friendship is tan 90°.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: exponents

Hi;

Welcome to the forum. It is a 3.

Answer is 3.

Last edited by bobbym (2013-02-13 00:47:27)

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

noelevans

Member

Registered: 2012-07-20

Posts: 236

Re: exponents

Or another way to look at it:

0th power of 12357 ends in 1.
1st power ends in 7.   2nd power ends in 9.   3rd power ends in 3.   4th power ends in 1
5th power ends in 7    6th power ends in 9    7th power ends in 3.    8th power ends in 1
9         ...            7    10      ...               9   11       ...              3    12    ...                  1, etc.

There are only four digits that occur in the powers and these repeat in the sequence 1, 7, 9, 3, 1, ...
Divide the power (655 in this case) by four and see what the remainder is.  It is three.  So the 655th
power ends in the same digit that the 3rd power does, namely 3. 

But this is a bit confusing since we get 3 for the remainder and for the last digit.
Try raising to the 656 power.  Then dividing 656 by 4 we get a remainder of zero.  So the 656
power ends in 1 same as the 0th power.

For the 654th power dividing 654 by 4 has remainder 2.  So the 654th power ends in 9 the same
as the second power.

For the 653rd power dividing 653 by 4 has remainder 1 so the units digit of the 653rd power is 7 the
same as the 1st, 5th, 9th, ... powers.

Be very blesses!

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Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).
LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

cooljackiec

Member

Registered: 2012-12-13

Posts: 186

Re: exponents

is noelevans levans from AoPS?

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I see you have graph paper.
You must be plotting something

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: exponents

Hi;

I do not think so. Noel is polite.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

niharika_kumar

Member

From: Numeraland

Registered: 2013-02-12

Posts: 1,062

Re: exponents

thnx guys,
epecially noel as m a student of class 9 and thus his soln ws much more easy fr me:).

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friendship is tan 90°.

noelevans

Member

Registered: 2012-07-20

Posts: 236

Re: exponents

Hi niharika_kumar,

You are quite welcome! Seeing patterns is often the name of the game in solving math problems.
And if you have access to a good math program you can often write a program to test for patterns
associated with a given problem.

Hi cooljackiec,  Not only am I not levans,  I have no clue as to what AoPS is.  What is it?

And thanks bobbym for the kind comment.  I do try to be polite and not hurt anyone's feelings
even if I think what they are saying is incorrect.  After all,  I have been wrong before.  As I
recall once in 1947 I was wrong about ...

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Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).
LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: exponents

Hi noelevans

AoPS is another forum. It's full name is Art of Problem Solving.

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

noelevans

Member

Registered: 2012-07-20

Posts: 236

Re: exponents

Thanks stefy!

I might have worried all night about what that meant.  I'll check the site out and see what it is about.

Bless you! 

---

Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).
LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: exponents

Hi

You're welcome (I think)!

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

noelevans

Member

Registered: 2012-07-20

Posts: 236

Re: exponents

Yes, you do think.  And you are quite good at it!

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Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).
LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: exponents

Depends on what you consider good.

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“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: exponents

You think therefore you am.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Mathegocart

Member

Registered: 2012-04-29

Posts: 2,226

Re: exponents

You think therefore you am.

such wise of a post.

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The integral of hope is reality.
May bobbym have a wonderful time in the pearly gates of heaven.
He will be sorely missed.