Question : What is the unit digit of $(217)^{413}\times (819)^{547}\times (414)^{624}\times (342)^{812}$?
Option 1: 2
Option 2: 4
Option 3: 6
Option 4: 8
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Correct Answer: 8
Solution :
Given:
(217)
413
× (819)
547
× (414)
624
× (342)
812
Solution:
The cyclicity of 7 is 4.
The cyclicity of 9 is 2.
The cyclicity of 4 is 2.
The cyclicity of 2 is 4.
According to the cyclicity theorem, divide the powers by 4 and take reminders. Otherwise, take 4.
The unit digit of (217)
413
= 7
413
= 7
(4 × 103) + 1
= 7
1
= 7
The unit digit of (819)
547
= 9
547
= 9
(2 × 273) + 1
= 9
1
= 9
The unit digit of (414)
624
= 4
624
= 4
(2 × 312)
= 4
4
= 6
The unit digit of (342)
812
= 2
812
= 2
(4 × 203)
= 2
4
= 6
The unit digit of the given expression will be the same as the unit digit of 7
413
× 9
547
× 4
624
× 2
812
⇒ 7
1
× 9
3
× 4
4
× 2
4
For unit digit ⇒ 7 × 9 × 6 × 6
$\therefore$ The required unit digit = 8
Hence, the correct answer is 8.
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