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Unofficial Section => Offtopic => Topic started by: kingW3 on April 07, 2013, 12:43

Title: Math problem
Post by: kingW3 on April 07, 2013, 12:43
Had similar problem on exam,took me a while to solve can you do it?
Solve for n where n is natural number
(http://img62.imageshack.us/img62/8649/50643491.jpg)
Title: Re: Math problem
Post by: courierpower on April 07, 2013, 13:51
multiply first term with 1 but as "(/2-1):(/2-1)" (lazy do download mathtype program i mean root 2 with /2), it will be /2-1 at end. Do the same calculation for each part. It will be like (/2-1) + (/3-/2) + (/4-/3) and u will see the rooted numbers remove each other, only -1 and the last term remains (/n+1) so, (/(n+1))-1=15, /(n+1)=16 n+1=256 n=255
Title: Re: Math problem
Post by: donjacrtasamir on April 07, 2013, 14:22
Had similar problem on exam,took me a while to solve can you do it?
Solve for n where n is natural number
(http://img62.imageshack.us/img62/8649/50643491.jpg)


lol people here get confused when they have to multiply 2 numbers and you think they can solve this....
Title: Re: Math problem
Post by: kingW3 on April 07, 2013, 14:28
Wanted to see who can solve this,courier solved it though.Nice job
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 15:12
Oh, I'm late.

By the way, fuck you, you made me spend 90 minutes trying to remember that little trick. I feel stupid. Especially because I did very similar exercise several yeras ago.
Title: Re: Math problem
Post by: courierpower on April 07, 2013, 15:13
in these kind of problems, i mean root number + or - sth else below the divide line, we multiply with their
eşlenik ifade
neutralizer or whatever you call i dont know math words in english so well,
for any term like"/x+/y" you just multiply that with "/x-/y" and it becomes "x-y" and you get rid of roots below the divide line. I could do these since 3 years ago when i was 8. grade. We do learn much stuff about math.
Title: Re: Math problem
Post by: kingW3 on April 07, 2013, 15:25
You rationalize,anyway this was in a test for mathematics high school(hardest) + these kind of questions are also in exams for faculty(college),i think you can solve it with Stolz theorem but not sure.
Btw if you're in the mood i'll continue,a easier one
120!
------
12120

Simplify it
Title: Re: Math problem
Post by: courierpower on April 07, 2013, 16:02
wasnt that "120!" factorial? or !120 is sth different?
Title: Re: Math problem
Post by: kingW3 on April 07, 2013, 16:03
Oops,mistype it's factorial
Title: Re: Math problem
Post by: courierpower on April 07, 2013, 16:26
Well all i found is that 2².3⁶² remains at the bottom dunno about top this wasn't easier  :-\
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 18:25
So stupid.
528 * 719 * 1110 * 139 * 177 * 195 * 235 * 295 * 313 * 373 * 412 * ... * 592 * 61 * ... * 113

2124*389

Note - all power bases in above fraction are prime.

TBH it doesn't look any simpler :D
Title: Re: Math problem
Post by: kingW3 on April 07, 2013, 18:46
I guess that's the answer since my friend(who told me that question just said it is something/2^124*3^89(at least i think),although i think it can be even shorter but meh :)

How much are there double-digit natural numbers where the result of addition of the digits doesn't change if we multiply it by:2,3,4,5,6,7,8,9
(for example
23=2+3; 23*2=46=4+6 which isn't the number we're looking for clearly)

A)0; B)1; C)2; D)3; E)4;
Title: Re: Math problem
Post by: Nistor on April 07, 2013, 19:01
just tell me why do i need this shit in my life?
i finished school for some water shit... and need to know this common... xD
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 19:13
It can't be simpler, because, as I said, I left only prime numbers :P Some powers might be off though. I'm almost sure there are a few.

Let's say we have a double digit number xy, x and y being digits. Let's also call n any of {2,3,4,5,6,7,8,9}. If xy solves our problem, it woud be true that

10x + y = n*10*x + n*y
that gives us

10*(n-1)*x = -(n-1)*y // :10*(n-1)
x = -y/10

only natural number that solves this equation would be
x=0
y=0
but that doesn't make double-digit number. Therefore there's no such numbers.

Note that
Quote
23=2+3; 23*2=46=4+6 which isn't the number we're looking for clearly)
bolded parts are complete bullshit :P 23!=5 and 46!=10

King, calcuate this (only pen, paper and your brain allowed - no calculators etc.):

423 134 * 846 267 - 423 133
----------------------------------------- = ?
423 133 * 846 267 - 423 134
Title: Re: Math problem
Post by: BLOODYALBOZ on April 07, 2013, 19:27
1.
Upper numbers are equal to lower ones (idk how is said in english) and when you divide a number with himself it will always give 1 (4/4=1)
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 19:31
What do you mean by equal? Because I'm not sure if you solved it, or just did not read carefully :D

I mean:

423 134 * 846 267 - 423 133
-----------------------------------------
423 133 * 846 267 - 423 134
Title: Re: Math problem
Post by: BLOODYALBOZ on April 07, 2013, 19:39
Damn i just gave a quick look :P

you are making me to get a pen and paper -.-" :P



here we go on a long way :P

423 133 * 846 267 = 358084340778
358084340778 - 423 133= 358083917645

so now we have 358083917645 / 423 133 * 846 267 - 423 134

Let's do same thing there :
423 133 * 846 267=358083494511
358083494511 - 423 134 = 358083071377

now it's 358083917645 / 358083071377 and this is something i can't make by hand :P
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 20:06
WAAAAAAAAAAIT, I misspelled this :/ There should be + sign below fraction

423 134 * 846 267 - 423 133
----------------------------------------- = ?
423 133 * 846 267 + 423 134

Anyway, did you seriously do the multiplication by hand? :P
Title: Re: Math problem
Post by: BLOODYALBOZ on April 07, 2013, 20:12
Yes i did :P i like math, but not rly good at it :-/

and fuck you for your misspell :P

now it's 1 as i said at first place -.-"
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 20:41
But there's still easier way to do it :P
Like, look at numbers:
423 133
423 134
846 267
rings any bell? Anything in common?

Or try that:
1996 * 199719971997 - 1997 * 199619961996 = ?
Title: Re: Math problem
Post by: kingW3 on April 07, 2013, 20:48
423 133 * 846 267 + 423 134
----------------------------------------- = 1
423 133 * 846 267 + 423 134



Btw what about 15?
1+5=6
and
(15*4)6+0=6

Not sure what i miss told
Anyway what we are looking
x+y=xn+ yn
Note
if it's 25 for example and it's (25*2)50=5+0 and not 4 + 10
Title: Re: Math problem
Post by: div.ide on April 07, 2013, 21:21
Oh, I see. I completely missed the point :P

I think you misread my equation, just like 2pac did :D
Again:
423 134 * 846 267 - 423 133
----------------------------------------- = ?
423 133 * 846 267 + 423 134

It will be one, but I was hoping someone will be able to show it in a clear way.
Title: Re: Math problem
Post by: kingW3 on April 07, 2013, 21:24
I didn't miss anything,but i guess i should have explained
423 133 *846 267 + 846 267 - 423 133    423 133 * 846 267 + 423 134
-------------------------------------------------- = ---------------------------------------
        423 133 * 846 267 + 423 134            423 133 * 846 267 + 423 134
Title: Re: Math problem
Post by: kingW3 on June 03, 2013, 20:19
I just did a few problems for my bro's admission,so wanted to post some here xD
1.If p is a prime number and 3p+1 and 5p+1 are also prime number,how much are there p numbers that satisfy the conditions?
2.Triange ABC on which AB is equal to 6cm and the opposite angle of the side(the ∠ACB angle) is equal to 150°,What is the area of the circumscribed circle on the ABC triangle.
If anyone wants help or explanation of the problem(due of my English :P) feel free to ask.
Title: Re: Math problem
Post by: BLOODYALBOZ on June 03, 2013, 21:33
I just did a few problems for my bro's admission,so wanted to post some here xD
1.If p is a prime number and 3p+1 and 5p+1 are also prime number,how much are there p numbers that satisfy the conditions?
2.Triange ABC on which AB is equal to 6cm and the opposite angle of the side(the ∠ACB angle) is equal to 150°,What is the area of the circumscribed circle on the ABC triangle.
If anyone wants help or explanation of the problem(due of my English :P) feel free to ask.

1. from [-1/3 to -1/5] :P
2. ps the circle is from inside part or outside part ( dat english :P ) and what you do you mean with area surface ?
Title: Re: Math problem
Post by: T-Bag on June 03, 2013, 22:10
I just did a few problems for my bro's admission,so wanted to post some here xD
1.If p is a prime number and 3p+1 and 5p+1 are also prime number,how much are there p numbers that satisfy the conditions?
2.Triange ABC on which AB is equal to 6cm and the opposite angle of the side(the ∠ACB angle) is equal to 150°,What is the area of the circumscribed circle on the ABC triangle.
If anyone wants help or explanation of the problem(due of my English :P) feel free to ask.

1. from [-1/3 to -1/5] :P
2. ps the circle is from inside part or outside part ( dat english :P ) and what you do you mean with area surface ?

What is the area of the circumscribed circle on the ABC triangle. -> outside circle and its area in cm2 (square centimeter)

so if Im right it should be 113.1cm2 (square centimeter)
Title: Re: Math problem
Post by: div.ide on June 03, 2013, 23:15
I'd guess you're right, T_Bag (fu, posted while I was drawing that shit :P).

And, there's exactly one p that satisfies conditions.
Title: Re: Math problem
Post by: T-Bag on June 03, 2013, 23:55
well, I hope coz diameter of circumscribed circle should be: c/sin γ (gamma) in this case (where c=|AB|), actually it works for any side {a/sin α (alpha) or b/sin β (beta)}.

so its: 6/sin150=12
so area of the circumcircle: πr2

btw regarding 1st question, it cant be:
Quote
1. from [-1/3 to -1/5]
as its not prime number  :D

but just to make sure, did u mean what is "p" or how many "p" (as Im not sure about this since there is no limit so u cant really reach infinity  :D) ... just quick one might be p=2.

3*2+1=7
5*2+1=11

2 & 7 & 11 are Prime Numbers
Title: Re: Math problem
Post by: div.ide on June 04, 2013, 00:44
I guess he meant "how many". Obvioudly 2 satisfies condition, but how would you prove it's the only one? Or that there's more? Or infinity? :P It's actually quite simple, just think of it.

Anyway, why did you resort to trigonometry? This particular one could be done by perceptive 5th grader understanding how angles work in circle, by noticing one thing.
Title: Re: Math problem
Post by: T-Bag on June 04, 2013, 01:15
I guess he meant "how many". Obvioudly 2 satisfies condition, but how would you prove it's the only one? Or that there's more? Or infinity? :P It's actually quite simple, just think of it.

Anyway, why did you resort to trigonometry? This particular one could be done by perceptive 5th grader understanding how angles work in circle, by noticing one thing.
why not to use trigonometry when its "IMO" good example and the easiest way-became to my mind as first type of solution...btw Im out of school for 8y so I forgot a lot of things :(

and about 1st question, thats why I asked what did he mean... btw p has to be even number so I cant figure any other then p=2 coz 0 (zero) isnt prime and any other even number isnt prime...so Im waitng for your answer.
Title: Re: Math problem
Post by: kingW3 on June 04, 2013, 06:59
I guess he meant "how many". Obvioudly 2 satisfies condition, but how would you prove it's the only one? Or that there's more? Or infinity? :P It's actually quite simple, just think of it.

Anyway, why did you resort to trigonometry? This particular one could be done by perceptive 5th grader understanding how angles work in circle, by noticing one thing.
T_Bag
Should have told ya only first 8 grade math,because with sine it's pretty easy but all solutions count :D.
div.ide Well also it requires knowing that the sum of the opposite angles of the cyclic quadrilateral is equal to 180,which is done at 8th grade(actually our school didn't even mention it but ok) or you done it differently?
Anyway few problems incoming
Title: Re: Math problem
Post by: courierpower on June 04, 2013, 07:00
I just did a few problems for my bro's admission,so wanted to post some here xD
1.If p is a prime number and 3p+1 and 5p+1 are also prime number,how much are there p numbers that satisfy the conditions?
2.Triange ABC on which AB is equal to 6cm and the opposite angle of the side(the ∠ACB angle) is equal to 150°,What is the area of the circumscribed circle on the ABC triangle.
If anyone wants help or explanation of the problem(due of my English :P) feel free to ask.
2 fits there, and no any other can it must be even number and only even prime is 2 :D
btw i want to share all prime numbers (but not 2,3) can be written as 6n+1 or 6n-1 which makes it eaiser to find them in some other problems :D
and the triangle y t bags solution really fast u create secondary perpendicular triangle from the C corner and see their circumscribed circles are the same then 6/sin150=x/sin90 x=12 so area is 144pi
Title: Re: Math problem
Post by: T-Bag on June 04, 2013, 07:04
I guess he meant "how many". Obvioudly 2 satisfies condition, but how would you prove it's the only one? Or that there's more? Or infinity? :P It's actually quite simple, just think of it.

Anyway, why did you resort to trigonometry? This particular one could be done by perceptive 5th grader understanding how angles work in circle, by noticing one thing.
T_Bag
Should have told ya only first 8 grade math,because with sine it's pretty easy but all solutions count :D.Well also it requires knowing that the sum of the opposite angles of the cyclic quadrilateral is equal to 180,which is done at 8th grade(actually our school didn't even mention it but ok) or you done it different?
As I mentioned above, Im out of school for 8 years, so I just used stuff what I remember (poor me that I dont remember 8th grade :) )
and regarding first post about p I used Logical Statements (or w/e its called) to make sure its only number 2.
Title: Re: Math problem
Post by: kingW3 on June 04, 2013, 08:52
Well you got that hyptoneuse is 12(x=12),and because the center of circumscribed circle is at the middle of the hypotenuse therefore r is equal to x/2 so r=6 and P=36pi,but nice way of doing it :) .About the p,i'll wait for div.ide he seemed like he wanted to say something about it :P
Anyway first
In a circle two normal chords(by normal i mean they form a angle of 90° degrees) cut each other to lines 3cm and 7cm.What is the distance from the center of circle and the dot where the two chords cut.
A picture cause my english :P ,and yeah i love circles :D
(http://imageshack.us/a/img29/9801/circlew.png)
2.If n is a natural number so that 1+2+3+...+n is a 3-digit number with same digits(111,222,333,what i mean by this).Then the sum of the digits on n is equal to?
3.When n is divided by 3 you get 2 as a leftover(n % 3= 2 modulo operator),when it's divided by 37 leftover is 22 how much is the leftover when dividing 111(n % 111)
Feel free to ask questions :P
Title: Re: Math problem
Post by: div.ide on June 04, 2013, 11:40
Visual representation of simplest way to deal with circumscribed triangle:
(http://i.imgur.com/eHmMfuU.png)



No, I don't have to say much about p, because
Only 2 fits there, and no any other number can. It must be even number, and only even prime is 2 :D
If you fix english, it's basically the answer. Any odd p would result in even numbers after operations, and gneraly even numbers are not prime by definition. Thus, we need odd prime p, and there's only one odd prime number = 2, which incidentally satisfies our conditions.



Strings problem is another simple constrution:
(http://i.imgur.com/T1asNcB.png)
Title: Re: Math problem
Post by: kingW3 on June 04, 2013, 11:52
Touche :)
Title: Re: Math problem
Post by: div.ide on June 05, 2013, 14:58
Quote
2.If n is a natural number so that 1+2+3+...+n is a 3-digit number with same digits(111,222,333,what i mean by this).Then the sum of the digits on n is equal to?
1+2+3+...+n = n(n+1)/2

111 = 3*37*1
222 = 3*37*2 etc.
let's call all those 3-digit numbers p
p = 3*37*k, k belongs to {1,2,...,9}

so, n(n+1)2 = 3*37*k // if these 2 numbers are equal, they must have same prime factors, thus
Case 1)
n=37
thus (n+1)/2 = 19 = 3k and  is natural -> no valid resuts
Case 2)
n+1 = 37
thus n/2 = 18 = 3k
k=6, p = 666 -> fits
n= 36
sum of digits = 9
Case 3)
n/2 = 37
n+1 = 75 = 3k
k = 15 -> out of range
Case 4)
n+1/2 = 37
you can guess from case 3 this won't fit either
So answer's 9, but it isn't that much of an elegant solution.

Quote
3.When n is divided by 3 you get 2 as a leftover(n % 3= 2 modulo operator),when it's divided by 37 leftover is 22 how much is the leftover when dividing 111(n % 111)

using the fact(s) that 3|111 and 37|111

1) 2 (mod 3) = 3k + 2 (mod 111), k (belongs to) {0,1,2,...,36}
2) 22 (mod 37) = 37k + 22, (mod 111), k (belongs to) {0,1,2}

so, 2) gives us 3 potential numbers: 22, 59, 81
22 = 3*7 + 1
59 = 3*19 + 2
81 = 3*27 + 0
Title: Re: Math problem
Post by: div.ide on June 05, 2013, 15:16
Aha, just realised how could I make it much shorter.

n(n+1)/2 = 37 * 3k -> n(n+1) = 37 * 2 *3k

and since 0<k<10, it's obvious you'd have to group it as 37 * 6k because no other combination would result in 2 consecutive numbers (n*[n+1]), considering k<10

Now choosing between 6k = 36 and 6k = 38 (k being prime) is easy.
Title: Re: Math problem
Post by: kingW3 on June 14, 2013, 21:44
Well,Good job :D
I shall start with a bit advanced problems
How much is the sum of all the solutions of the equation:
x + sqrt(x^2+16)=40/sqrt(x^2+16)

Value of a real parameter m for which the sum of the square of the solutions of equation x^2-mx+m-3=0 smallest value of m is