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barxatty [35]
3 years ago
5

Show that an integer is divisible by 11 if and only if the alternating sum (add first digit, subtract the second, add the third,

subtract the fourth etc) of its digits is divisible by 11
Need an answer by tomorrow if possible please
Mathematics
1 answer:
Bess [88]3 years ago
3 0

Answer and Step-by-step explanation:

Suppose that we have a number y which is a positive integer and that:

y = x_n...x_5x_4x_3x_2x_1x_0

Where;

x_{0} = digit at 10⁰ => one's place (units place)

x_1 = digit at 10¹ => 10's place (tens place)

x_{2} = digit at 10² => 100's place (hundreds place)

x_{3} = digit at 10³ => 1000's place (thousands place)

.

.

.

x_{n} = digit at 10ⁿ place

Then;

y = x_{0} * 10⁰ + x_1 * 10¹ + x_{2} * 10² + x_{3} * 10³ + x_{4} * 10⁴ + x_5 * 10⁵ + . . . + x_{n} * 10ⁿ

<em>Since 10⁰ = 1, let's rewrite y as follows;</em>

y = x_{0}  + x_1 * 10¹ + x_{2} * 10² + x_{3} * 10³ + x_{4} * 10⁴ + x_5 * 10⁵ + . . . + x_{n} * 10ⁿ

Now, to test if y is divisible by 11, replace 10 in the equation above by -1. Since 10 divided by 11 gives -1 (mod 11)     [mod means modulus]

y = x_{0}  + x_1 * (-1)¹ + x_{2} * (-1)² + x_{3} * (-1)³ + x_{4} * (-1)⁴ + x_5 * (-1)⁵ + . . . + x_{n} * (-1)ⁿ

=> y =  x_{0}  - x_1 + x_{2} - x_{3} + x_{4} - x_5 + . . . + x_{n} (-1)ⁿ (mod 11)

Therefore, it can be seen that, y is divisible by 11 if and only if alternating sum of its digits x_{0}  - x_1 + x_{2} - x_{3} + x_{4} - x_5 + . . . + x_{n} (-1)ⁿ is divisible by 11

<em>Let's take an example</em>

Check if the following is divisible by 11.

i. 1859

<em>Solution</em>

1859 is divisible by 11 if and only if the alternating sum of its digit is divisible by 11. i.e if (1 - 8 + 5 - 9) is divisible by 11.

1 - 8 + 5 - 9 = -11.

Since -11 is divisible by 11 so is 1859

ii. 31415

<em>Solution</em>

31415 is divisible by 11 if and only if the alternating sum of its digit is divisible by 11. i.e if (3 - 1 + 4 - 1 + 5) is divisible by 11.

3 - 1 + 4 - 1 + 5 = 10.

Since 10 is not divisible by 11 so is 31415 not divisible.

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