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If n is a positive integer, what is the remainder when 38n+3 + 2 is divided by 5? 
  • a)
    1
  • b)
    2
  • c)
    3
  • d)
    4
Correct answer is option 'D'. Can you explain this answer?
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If n is a positive integer, what is the remainder when 38n+3 + 2 is di...
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If n is a positive integer, what is the remainder when 38n+3 + 2 is di...
Solution:

We need to find the remainder when 38n^3 - 2 is divided by 5.

Let's first simplify the expression 38n^3 - 2.

38n^3 - 2 = (40n^3 - 2n^3) - 2
= 2n^3(20 - 1) - 2
= 2n^3(19) - 2

Now, we need to find the remainder when 2n^3(19) - 2 is divided by 5.

Let's consider the remainders of 2n^3 and 19 when divided by 5.

When 2n^3 is divided by 5, the remainder can only be 0, 2, or 3.

When 19 is divided by 5, the remainder is 4.

Using these remainders, we can find the remainder when 2n^3(19) - 2 is divided by 5.

Case 1: When 2n^3 leaves a remainder of 0 when divided by 5

In this case, 2n^3(19) - 2 leaves a remainder of (-2) or 3 when divided by 5.

Case 2: When 2n^3 leaves a remainder of 2 when divided by 5

In this case, 2n^3(19) - 2 leaves a remainder of (2x4 - 2) or 6 when divided by 5.
But 6 leaves a remainder of 1 when divided by 5.

Case 3: When 2n^3 leaves a remainder of 3 when divided by 5

In this case, 2n^3(19) - 2 leaves a remainder of (3x4 - 2) or 10 when divided by 5.
But 10 leaves a remainder of 0 when divided by 5.

Therefore, the possible remainders when 2n^3(19) - 2 is divided by 5 are 0, 1, or 3.

But we know that the correct answer is option 'D' or 4.

So, the only possible remainder left is 4.

Hence, the remainder when 38n^3 - 2 is divided by 5 is 4.
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Community Answer
If n is a positive integer, what is the remainder when 38n+3 + 2 is di...
The units digit of 3 in positive integer power has cyclicity of 4 for the unis digit:
31 --> the units digit is 3;
32 --> the units digit is 9;
33 --> the units digit is 7;
34 --> the units digit is 1;
35 --> the units digit is 3 AGAIN;
...

So, the units digit repeats the following pattern {3-9-7-1}-{3-9-7-1}-.... 38n+3 will have the same units digit as 33, which is 7 (remainder when 8n+3 divided by cyclicity 4 is 3). Thus the last digit of 38n+3+2 will be 7+2=9. Any positive integer with the unis digit of 9 divided by 5 gives the remainder of 4.
 
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