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ExpertsGlobal5
What is the digit at the units place in the expanded form of 27^38 ?

A. 1
B. 3
C. 6
D. 7
E. 9


­
We need to find the UNITS digit of 27^38.

7 powers follow a pattern.

7^1 = 7

7^2 = 49 ( unit digit 9)

7^3 = 343 ( unit digit 3)

7^4 = 2401 (unit digit 1)

7^5 = (2401*7) = xxxxx7 (unit digit is again 7)

The cycle repeats after every 4 numbers.

We are asked for 7^38 ?

38 divided by 4 = (9*4)+ 2

Where 2 is a remainder.

So, there are 9 full cycles of (7————> 9 ————> 3 ————> 1).

Then additional increment of 2. Which lands in 7————> 9 ————> 3 ————> 1

9

Option E
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Only the units digit of the base (7 in 27) affects the units digit of powers.

The power of 7 repeated it's unit digit in a cycle of 4
i.e. 7, 9, 3, 1

So computing 38 mod 4, we get a remainder of 2

So 27^38 has the same unit digit as of 7^2, that is 9

So the correct option is E
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This can be found using cyclicity property
Since its 27^38, we must use cyclicity of 7- 7,9,3,1
Since power is 38 we get reminder as 2 which corresponds to digit 9
Hence option E
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What is the digit at the units place in the expanded form of 27^38 ?

A. 1
B. 3
C. 6
D. 7
E. 9

Experts' Global Video Explanation:


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