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Prove 4•10^(2n)+9•10^(2n-1)+5 is divisible by 99 for n is in N.
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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.
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On the "divisibility by 11 rule" see the lesson
- Divisibility by 11 rule
in this site.
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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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