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prove that if a decimal number is divisible by 3 if and only if the sum of its digits is divisible by three

Sagot :

if a decimal number is divisible by 3 if and only if the sum of its digits is divisible by three is evident and proved below.

Let n be a decimal number.

Assume n is divisible by 3.

lets consider n in terms of its digits as

n = d_1 * 10^(m-1) + d_2 * 10^(m-2) + ... + d_m * 10^0

where m is the number of digits in n and d_1, d_2, ..., d_m are its digits.

Substituting n = 3k into the above equation, we get

3k = d_1 * 10^(m-1) + d_2 * 10^(m-2) + ... + d_m * 10^0

Subtracting 3k from both sides of the equation, we get

0 = d_1 * 10^(m-1) + d_2 * 10^(m-2) + ... + d_m * 10^0 - 3k

Adding 3k to both sides of the equation, we get

3k = d_1 * 10^(m-1) + d

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