SOLUTION: Prove 4•10^(2n)+9•10^(2n-1)+5 is divisible by 99 for n is in N.

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Question 1179135: Prove 4•10^(2n)+9•10^(2n-1)+5 is divisible by 99 for n is in N.
Found 3 solutions by greenestamps, MathLover1, ikleyn:
Answer by greenestamps(13200)   (Show Source): You can put this solution on YOUR website!


Use mathematical induction....

(1) show it is true for n=1; then (2) show that if it is true for n=k then it is also true for n=k+1

(1) When n=1, the expression is 4(100)+9(10)+5 = 495 = 5(99).

The expression is divisible by 99 for n=1.

(2) Assume the expression is true for n=k and show that it follows that it is true for n=k+1:

Assume 4*10^(2k)+9*10(2k-1)+5 is divisible by 99. Then

4*10^(2(k+1))+9*10^(2(k+1)-1)+5 =
4*10^(2k+2)+9*10^(2k+1)+5 =
100(4*10^(2k)+9*10(2k-1)+5)-495

In that expression, (4*10^(2k)+9*10(2k-1)+5) is divisible by 99 and 495 is also divisible by 99, so we have shown that if the expression is true for n=k then it follows that it is true for n=k+1.

That completes the proof by mathematical induction.


Answer by MathLover1(20850)   (Show Source): You can put this solution on YOUR website!

Assuming that


=
=
=
=
=
=
= => is a factor, means it is divisible by


Answer by ikleyn(52785)   (Show Source): You can put this solution on YOUR website!
.
Prove 4•10^(2n)+9•10^(2n-1)+5 is divisible by 99 for n is in N.
~~~~~~~~~~~~~~

The given number is


    .    (1)



    +---------------------------------------------------------------+
    |    Notice that the number    gives the remainder 1,      |
    |    when is divided by 9, for ANY positive integer m.          |
    +---------------------------------------------------------------+



It implies that the first addend in  (1)  gives the remainder 4, when is divided by 9.

The second addend in  (1)  is divisible by 9 without the remainder (OBVIOUSLY).


    It makes it OBVIOUS that the number (1) is divisible by 9 without the remainder.



Next, the number (1) has the form  49000 . . .005, where 

    - the leading digit 4 is located in some O D D position (2n+1), counting from the most right "ones" position;

    - the digit 9 is placed in the next (E V E N) position (2n);

    - and the last, "ones" digit 5 is located in the most right position number ONE (counting from the right);

    - while all the other digits are zeros.


Applying the "divisibility by 11 rule", we see that the number (1) has alternate sum of digits  4 - 9 + 5 = 0.


        Since the alternate sum of digits equal 0 (i.e. is divisible by 11),
        it means that the number (1) itself is divisible by 11,
        according to the "divisibility by 11 rule.



Thus the number (1) is multiple of 9 and 11 --- HENCE, it is multiple of 99.

Solved (mentally), explained and completed.

------------------

On the "divisibility by 11 rule" see the lesson
    - Divisibility by 11 rule
in this site.


        *******************************************************
                An integer number is divisible by  11  if and only if
                the alternate sum of its digits is divisible by  11.
        *******************************************************



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