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Algebra Level 3

2011 2012 × 2013 2014 \large { \color{#D61F06}{2011} }^{ \color{teal}{2012} } \times { \color{#D61F06}{2013}}^{ \color{teal}{2014} }

Find the number of digits of the number above when stated in decimal representation.

Details and Assumptions

You may use the following approximations

  • log 10 2011 = 3.3034120705967 \log_{10} 2011 = 3.3034120705967 \ldots

  • log 10 2013 = 3.3038437748886 \log_{10} 2013 = 3.3038437748886 \ldots


The answer is 13301.

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1 solution

Somoy Subandhu
Nov 17, 2014

Using Logarithms we can say that, log 2011 2012 × 2013 2014 + 1 \left\lfloor \log { { 2011 }^{ 2012 }\times { 2013 }^{ 2014 } } +1 \right\rfloor

or, log 2011 2012 + log 2013 2014 + 1 \left\lfloor \log { { 2011 }^{ 2012 }+\log { { 2013 }^{ 2014 } } } +1 \right\rfloor

or, 2012 log 2011 + 2014 log 2013 + 1 \left\lfloor 2012\log { { 2011 }\quad +\quad 2014\log { { 2013 } } } +1 \right\rfloor

hence, we can find the answer , 13301

P.s: If you have questions please comment below

Yes, I used the same method.

Chew-Seong Cheong - 6 years, 6 months ago

i did not understood the whole theory behind your formula. can you explain it to me.

aditya vikram - 6 years, 6 months ago

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P.S: Here , Every log's base is 10. Note that, log 10 1 = 0 \log _{ 10 }{ 1\quad =\quad 0 } and, log 10 10 = 1 \log _{ 10 }{ 10\quad =\quad 1 } also , log 10 100 = 2 \log _{ 10 }{ 100\quad =\quad 2 }

Have you noticed Something? when the number of digits were 1 the result of it's log was 0.

when the number of digits were 2 the result of it's log was 1. and

when the number of digits were 3 the result of it's log was 2.

have you noticed a pattern ? we can measure any number's number of digits using log !

now let us Create a Formula !

I want to find out how many digits are there in the number '1' we can easily tell it's 1 but we should find it using log!

for log 1=0 so, if we add one with it we can find the number of the digit! log 1 + 1 =1 ! now let's try it for 10 and 100.

log 10= 1 hence, log 10+1= 2 , So 10 has 2 digits.

log 100 =2 hence, log 100+1 = 3 , so 100 has 3 digits.

Now, What if the number is something like 2 or 5 or 126 or something?

log 2 = 0.301.. hence log 2+1=1.301... but it has 1 digit! so how can we make 1.301... into 1? we use "floor" ofcourse !! floor is a symbol like this x \left\lfloor x \right\rfloor It means the highest integer which is smaller than x. like 2.5 = 2 , 2.11 = 2 a n d 2.9999 = 2 \left\lfloor 2.5 \right\rfloor =2\quad ,\quad \left\lfloor 2.11 \right\rfloor =2\quad and\quad \left\lfloor 2.9999 \right\rfloor =2

so now we can say, log 10 2 + 1 = 1 \left\lfloor \log _{ 10 }{ 2\quad +\quad 1 } \right\rfloor =1

the same rule applies for all positive integer numbers ! So now we can simply say that , for any natural number 'N' , the number of digits N has is log 10 N + 1 \left\lfloor \log _{ 10 }{ N\quad +\quad 1 } \right\rfloor

Now look at my solution once more ! If you still have problem understanding it let me know!

Somoy Subandhu - 6 years, 6 months ago

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Thanks very much for your explanation, but could you explain me how you calculate the log of 2011 and 2013 without the use of a calculator. Thanks.

Jesus Ulises Avelar - 6 years, 6 months ago

Great explanation...!!!

Ankan Poddar - 6 years, 6 months ago

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@Ankan Poddar Thanks ^_^

Somoy Subandhu - 6 years, 6 months ago

It's like this for anti-log there are two main parts.. The one before the decimals and the ones after the decimals.. Consider the log value is x.y (. represents decimal and not multiplication), after anti-log y gives the ans & (x+1) gives the number of digits after which the decimal is to be placed..

Ra Ka - 6 years, 6 months ago

can u plz elaborate it am so bad at maths :P

Fatima Saif - 6 years, 6 months ago

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Have you checked my reply/response to Aditya Vikram's comment ? If you didn't please check ! After checking if u still have troubles understanding it tell me

Somoy Subandhu - 6 years, 6 months ago

really nice explanation

Sandeep KV - 6 years, 6 months ago

i dint undrstood dat theory .....cn u xplain clearly

Vikas Viky - 6 years, 6 months ago

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Have you checked my reply/response to Aditya Vikram's comment ? If you didn't please check ! After checking if u still have troubles understanding it tell me

Somoy Subandhu - 6 years, 6 months ago

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