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Algebra Level 3

Let k= (2012)^2 + 2^2012 , then last two digit of the equation is


The answer is 40.

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4 solutions

Sunil Pradhan
Mar 25, 2014

2012^2 last 2 digits = 44

2^2012 = 2^10^200 × 2^10 × 2^2

= 1024^200 × 1024 × 4

24^ even power ends in 76 and 24^odd power ends in 24

total of last 2 digits = 44 + (76 × 24 × 4) = 44 + 76 × 96 = 44 + 96 = 40 considering only last 2 digits,

2012 × 2012 2012 \times 2012 leaves 44 44 such the last two digits. Well, for 2 2012 2^{2012} , we should see the the numbers in the form 2 n 2^n : 2 1 = 2 , 2 2 = 4 , 2 3 = 8 , 2 4 = 16 , 2 5 = 32 , 2 6 = 64 , 2 7 = 128 , 2 8 = 256 , . . . , 2 12 = 4096... 2^1=2, 2^2=4, 2^3=8, 2^4=16, 2^5 = 32, 2^6 = 64, 2^7=128, 2^8 = 256, ..., 2^{12} = 4096... We can realize that all the numbers in the form 2 4 m 2^{4m} have 6 6 as units digit. The tens digit grows in an arithmetic progression, with a common difference equal to 4 4 , and the first term is 1 1 . So:

2 2012 = ( 2 4 ) 503 2^{2012} = (2^4)^{503} a n = 1 + 502 4 a_n = 1 + 502\cdot 4 a n = 1 + 2008 = 2009 a_n = 1 + 2008 = 2009

So, the tens digit is 9 9 .

Finally, 44 + 96 = 140 44 + 96 = 140 , and the last two digits is 40 40 .

What is only sought is the last two digits of the sum, so let us only look for the last two digits of both 201 2 2 2012^2 and 2 2012 2^{2012} .

For 201 2 2 2012^2 (we can just ignore the leftmost numbers),

2 0 1 2
2 0 1 2
-------
    2 4
    2
-------
    4 4

So, the last two digits of 201 2 2 2012^2 is 44 44 .

Next, we have to find the last two digits of 2 2012 2^{2012} . We will start with 02 02 , which is 2 2 , just with a leading 0 0 in the tens place. The zero will not matter anyway since 0 × 2 = 0 0 \times 2 = 0 , that is, the tens digit remains a zero. If we are going to multiply 02 02 by itself long enough, taking note of only the last two digits, you would see a pattern.

1 s t t e r m = 02 2 n d t e r m = 04 3 r d t e r m = 08 4 t h t e r m = 16 5 t h t e r m = 32 6 t h t e r m = 64 7 t h t e r m = 28 8 t h t e r m = 56 9 t h t e r m = 12 1 0 t h t e r m = 24 1 1 t h t e r m = 48 1 2 t h t e r m = 96 1 3 t h t e r m = 92 1 4 t h t e r m = 84 1 5 t h t e r m = 68 1 6 t h t e r m = 36 1 7 t h t e r m = 72 1 8 t h t e r m = 44 1 9 t h t e r m = 88 2 0 t h t e r m = 76 2 1 s t t e r m = 52 2 2 n d t e r m = 04 2 3 r d t e r m = 08 \begin{aligned} 1^{st} term &= 02 \\ 2^{nd} term &= 04 \\ 3^{rd} term &= 08 \\ 4^{th} term &= 16 \\ 5^{th} term &= 32 \\ 6^{th} term &= 64 \\ 7^{th} term &= 28 \\ 8^{th} term &= 56 \\ 9^{th} term &= 12 \\ 10^{th} term &= 24 \\ 11^{th} term &= 48 \\ 12^{th} term &= 96 \\ 13^{th} term &= 92 \\ 14^{th} term &= 84 \\ 15^{th} term &= 68 \\ 16^{th} term &= 36 \\ 17^{th} term &= 72 \\ 18^{th} term &= 44 \\ 19^{th} term &= 88 \\ 20^{th} term &= 76 \\ 21^{st} term &= 52 \\ 22^{nd} term &= 04 \\ 23^{rd} term &= 08 \\ \vdots \end{aligned}

Starting from the 2 n d 2^{nd} term, we will have a repeating sequence of numbers. The sequence repeats every 20 20 terms, that is, the number repeats in the 2 n d 2^{nd} term, 2 2 n d 22^{nd} term, 4 2 n d 42^{nd} term, and so on. We can generalize this pattern by saying that, except for the first term, for all positive integers k k , all ( 20 n + k ) t h (20n + k)^{th} terms are the same. Now, this implies that the 201 2 t h 2012^{th} term is the same as the 1 2 t h 12^{th} term, which is 96 96 .

Thus, the last two digits of the sum in question is 44 + 96 40 44 + 96 \equiv \boxed{40} .

Bart Khau
Mar 11, 2014

2^22 has 04 as last 2 digits

so 2^2002= 2^(91*22) will have 04 as unit digits

2^2012=(2^2002) (2^10)= 04 1024= 96 as last 2 digits

add 2012^2 to it 40 is last two digits.

In the problem , it should be number and not equation.

Rajarshi Das - 7 years, 3 months ago

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