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# which digit doesn't occur in this number?

I came across this question on a site and I didn't find any solution to this.....(except to use calculator)

The question was

Which digit doesn't occur in the number 2^29 ?

Plz help.....

Note by Poonayu Sharma
3 years, 7 months ago

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If 2^29=n (mod 9),then 9-n is the answer. Noticing that 2^3=-1(mod 9),2^29=(2^3)^9x2^2=(-1)^9x2^2=-4=5 (mod 9). Therefore the answer is 9-5=4. This can be easily derived using mod. Hope you got it :)

- 3 years, 7 months ago

Are there other ways too?

- 3 years, 7 months ago

Sorry, I can't fathom any. :(... I'm really bad at intuition :-/

- 3 years, 7 months ago

Ohh...use of mod didn't strike me :P

I was trying to solve it using logs etc. . :P

Btw...thanks you sir

- 3 years, 7 months ago

You're welcome :D....but you're 3 yrs elder, so you must refrain from calling me sir, how could you? And this solution is also inspired by an experiment I did with the cyclicity of digital roots of powers of 2. You could check out my easy-peasy problem on that though :)

- 3 years, 7 months ago

U deserved "sir" :D.....and I solved the problem u talking about....but not with the (mod 9) method But with cyclic sum of digits of powers of 2

- 3 years, 7 months ago

Oh. Well at first ( when I encountered this problem in class 7) I solved this using cyclicity too :)

- 3 years, 7 months ago

Ohh...well dont u think it becomes a herculean task for a 7 grade guy to do such sums :P

- 3 years, 7 months ago

@Poonayu Sharma - Woah! I'm mind -blown!!!! How did you level up in Algebra and Number Theory so soon? What resources did you use to gain here?

- 3 years, 6 months ago

I saved the challenging questions for later. ..then I read all the posts or notes related to them..For some.I used Google to search theories related to sum..and then tried to solve them....though I couldn't solve all(surprisingly I was able to solve 3 lvl 5 question) ;P ..I guess you can also do it ...faster than me :D

Next target ....JEE ,CALCULUS AND MECHANICS :P

- 3 years, 6 months ago

What did you use to learn all those algebra question- inequalities , Symmetric bounding,,all that. And sorry, I really dont think I can do it faster :(.

- 3 years, 6 months ago

Frankly speaking. ...The symmetric bounding question was a hit and trial by me..(I think one should not hesitate to use hit and trial if there are no other means at times.).I got it in the second try..

.whenever u see a problem. ..try to find a note about it .(That's the only thing I used to crack sums..also me and my 2 friends did few sums together)..I remember daniel liu once gave a link to a note which could be used to solve a problem...It was useful then ..If u don't find any such notes...Google it...believe me problems do become easier then.. :D Hoping ur level soars high :P

- 3 years, 6 months ago

:D

- 3 years, 6 months ago

Btw...u planning to do iit? :P

- 3 years, 6 months ago

Nope. I do math only as a hobby. I'd wan't to take up medicine as my mother wants me to do it. U wanna go to IIT or somewhere else Mr. Future Cosmologist?

- 3 years, 6 months ago

I am bad at bio and chemistry right from birth :P..I will do cosmology (wanna go to NASA) whether I get iit or not..currently I'm preparing for iit ...let's see if luck shines upon me and if i get a good rank then ill do iit ...well if not then I'm thinking for PhD in mathematics and then cosmology ( possibly astronaut too) :P

But the way medical ppl will have a tough competition because of u :P

- 3 years, 6 months ago

Nice :). I also loved the way you ended it with a joke :). You want to become an astronaut or cosmologist? I must say there is a great difference between the two! But, whatever it be I'm sure you'd do well in it :P. You are preparing for IIT on your own? I feel ICSE curriculum is much better than CBSE. What do you feel?

- 3 years, 6 months ago

I want to become both actually :P....I have joined classes here called pace for preparation. .I myself have studied from icse and I feel it's standard is better than other boardr...But while you prepare for iit ..It doesn't matter which board ur from ...All become more or less same

Thanks for ur compliment though ...

And good luck. .. (remember to enjoy life as much as u can now...After 10 ...its hell :( :P)

- 3 years, 6 months ago

YUP. Why do you say life after 10th is hell? In fact, I would consider that heaven. I would have to study only my favorite two subjects. English is passable too. In fact, life now is hell for me!.

- 3 years, 6 months ago

I used to think the same way bro....You will realise it after u leave ur school and join college or classes ...everything suddenly gets screwed up :P
but if u manage ur time and other things (which I failed to)...u wont have troubles .. Hope you enjoy it even after school :D

- 3 years, 6 months ago

Oh! Thanks for your well wishes :)

- 3 years, 6 months ago

You can view the question here with a detailed solution discussion.

Staff - 3 years, 7 months ago

Except there you are given that it's a 9 digit number with distinct digits. The first step in this problem is to show that it is a 9 digit number (using log) and then obviously by the way the problem is stated implies that the digits are distinct

- 3 years, 7 months ago

That rises an interesting question. Which powers (or 2) are missing exactly 1 digit (but could have repeats of others)? Unfortunately, I don't think there is an easy way to answer this question.

Staff - 3 years, 7 months ago

Yeah I forgot to mention it because I thought that its understood :P

- 3 years, 7 months ago

Thank you sir :D

- 3 years, 7 months ago