This question is odd

As n n ranges over all real values in the interval [ 0 , 100 ] [0, 100] , how many values of n n are there such that 4 n + 1 4n+ 1 is an odd integer?


The answer is 201.

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19 solutions

Daniel Chiu
Dec 21, 2013

Let f ( n ) = 4 n + 1 f(n)=4n+1 . Note that f ( n ) f(n) is continuous and monotonically increasing (never decreases). Also, f ( 0 ) = 1 f(0)=1 and f ( 100 ) = 401 f(100)=401 , so every odd number from 1 to 401, inclusive, is achieved exactly once for n n in the interval [ 0 , 100 ] [0,100] . This is 201 \boxed{201} values.

"4n+1" is always odd whatever the the value of "n". our boundary for "n" is 1 to 100.so there are 100 values of "n" for which "4n+1" is odd..... the answer should be 101..... do correct me if I am wrong.... where is the problem..... ???

Anom Ahmed - 7 years, 5 months ago

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The problem is that nonintegers can work too. For example, n = 1 2 n=\dfrac{1}{2} gives 4 n + 1 = 3 4n+1=3 , which is an odd integer.

Daniel Chiu - 7 years, 5 months ago

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u re goood

math dude - 7 years, 2 months ago

There are infinite real numbers n between 0 and 100 that satisfy the question.. what's about 17/2?

Andrea Virgillito - 5 years, 1 month ago

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@Andrea Virgillito well actually, only multiples of 1/2 satisfy this question and as you know, there are 201 multiples of 1/2 from 0 to 100 inclusive.

Faisal Mujawar - 2 years, 1 month ago

Naming the variable "n" may not have been the best letter to use for this problem. It is common practice for "n" to be an integer. I think that may have threw you off, Anom.

Ken Hodson - 5 years, 7 months ago

Also, 4 n + 1 4n + 1 isn't always odd. For example, n = 1 4 n = \frac {1}{4} gives 2 2 as the result, and 1 4 \frac {1}{4} is included on the interval given.

Diego E. Nazario Ojeda - 7 years, 5 months ago

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Also, 4 n + 1 4n+1 isn't always an integer. For example, n = 0.1 , 1.11... , e , e t c . n=0.1, 1.11..., e, etc. because if n n is in real number, 4 n 4n is also in R. And we can say that 4 n + 1 4n+1 is in R.

Samuraiwarm Tsunayoshi - 7 years, 5 months ago

rite anom..... plz Daniel elaborate

Prakash Tiwari - 6 years ago

Yes I agree with you.. All the vales of "4n+1" would be odd, and so the answer should be 101.

Gayatri Gade - 5 years, 6 months ago

daniel, could you give some tips on how to think like this.Or to produce insightful solutions?

A Former Brilliant Member - 7 years, 5 months ago

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I thought all "real" so I thought only the real numbers, my answer is 101.

DÉSIRE DÉSIRE - 2 years, 5 months ago

Good explanation, thanks!

Mira B - 7 years, 5 months ago

Nice solution!

Vishnu Kulkarni - 7 years, 5 months ago

This is elegancy

Juan rodrígez - 7 years, 5 months ago

What an elegant answer Daniel! Hats off to you

Ankit Chabarwal - 4 years, 9 months ago

it has not stated that n is integer or a rational no..

suresh jh - 3 years, 4 months ago

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No it states that n is a real number. That's why Daniel can use a continuous and monotonous function. On that line only rationals (0/2, 1/2, 2/2 etc) satisfy the condition and he rationals are a subset of the reals.

Maarten van Helden - 3 years ago

Simply stated: For each whole number from 0 to 100 the result is odd. Count 101.

For n=.5, 1.5, 2.5... 99.5, the result is also odd. Count 100.

For any other value of n, n, 4n an 4n+1 all result in a fraction, not an odd number. Count = 0.

So... 101 +100 = 201. The answer.

James Schuller - 2 years, 3 months ago

It says n is real, not an integer, so it's not 101, it's 201.

Andrew Carratu - 9 months, 1 week ago

As n n ranges over all real values in the interval [ 0 , 100 ] [0,100] , we need to find out for how many values of n n is 4 n + 1 4n+1 an odd integer. For 4 n + 1 4n+1 to become an integer, 4 n 4n needs to be an integer.If n n is an integer then definitely 4 n 4n is an integer since multiplication of integers follows the closure property. We know that, 2 4 2 \mid 4 2 4 n \Rightarrow 2 \mid 4n Thus 4 n 4n is an even integer. But when we add 1 1 to 4 n 4n it becomes an odd integer i.e; 2 4 n + 1 2 \nmid 4n+1 So, every integer of the form 4 n + 1 4n+1 is an odd integer, where n ϵ Z n \epsilon Z . In the interval [ 0 , 100 ] [0,100] , there are 101 101 integers, thus there are 101 101 odd integers of the form 4 n + 1 4n+1 in this interval. But, as n n ranges over all real values in the given interval, 4 n 4n can also be an integer when n n is of the form 4 k + 1 4 \frac{4k+1}{4} , 4 k + 2 4 \frac{4k+2}{4} or 4 k + 3 4 \frac{4k+3}{4} for some k ϵ Z k \epsilon Z .
Now we need to find out of these three type of real numbers how many give us an even integer for 4 n 4n . Since if 4 n 4n is even then only 4 n + 1 4n+1 will be odd. C a s e 1 : Case 1: 4 × 4 k + 1 4 4 \times \frac{4k+1}{4} = 4 k + 1 =4k+1 It is an odd integer since 2 4 k + 1 2 \nmid 4k+1 . C a s e 2 : Case 2: 4 × 4 k + 2 4 4 \times \frac{4k+2}{4} = 4 k + 2 =4k+2 = 2 ( 2 k + 1 ) =2(2k+1) It is an even integer since 2 2 ( 2 k + 1 ) 2 \mid 2(2k+1) . In this problem the value of n n i.e 4 k + 2 4 \frac{4k+2}{4} should be in the inteval [ 0 , 100 ] [0,100] . There are 100 such real numbers ranging from 1 2 \frac{1}{2} to 199 2 \frac{199}{2} . Thus in this case we get 100 100 odd integers of the form 4 n + 1 4n+1 in the given interval. C a s e 3 : Case 3: 4 × 4 k + 3 4 4 \times \frac{4k+3}{4} = 4 k + 3 =4k+3 = 2 ( 2 k + 1 ) + 1 =2(2k+1)+1 = 2 k + 1 =2k^{'}+1 for some k ϵ Z k^{'} \epsilon Z . It is an odd integer since 2 2 k + 1 2 \nmid 2k^{'}+1 = 2 4 k + 3 =2 \nmid 4k+3 Thus, there are no even integers in this case also. Therefore there are a total of 101 + 100 101+100 = 201 =\boxed{201} Thus there are a total of 201 \boxed{201} odd integers of the form 4 n + 1 4n+1 when n n ranges over all real numbers in the interval [ 0 , 100 ] [0,100] .

It says n is real, not an integer, so it's not 101, it's 201.

Andrew Carratu - 9 months, 1 week ago
Davin Leo
Dec 21, 2013

4 n must be an even integer since even + odd = odd

4 n = 2 k
n = 0.5 k where k is an integer with interval [0,200]
There are 201 numbers from 0 to 200

Therefore, there are 201 values of n such that 4 n + 1 is an odd integer

N=1/4. 4n+1=1. So your solution is not correct.

Bibolar Vlad - 4 years, 6 months ago

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Nevermind. My Bad.

Bibolar Vlad - 4 years, 6 months ago

4n+1 = 2 when n=1\4

A Former Brilliant Member - 2 years, 1 month ago

Nope. For n=1/4, 4n+1=2 (which is even!)

James Schuller - 2 years, 3 months ago

It's from 0 to 100, not 200

Hossain Nahdi - 3 months, 1 week ago

most helpfull among all

SHASWAT ANAND - 2 weeks, 6 days ago
Habib Issa-sy
Jan 8, 2016

n ranges between 0 and 100, so 4n+1 ranges between 1 and 401, and there is 201 odd integer in this interval!

Moderator note:

Good explanation. The trick here is that n n need not be an integer.

Very elegant and concise solution. I worked out that if n was an integer 4n + 1 would be odd and that if n ended in .5 4n+1 would also be odd and that there were 201 values that fitted these criteria (101 integers and 100 intermediate values).

Thomas Sutcliffe - 3 years, 5 months ago
Janil Garcia
Jun 11, 2015

We know that 4 n + 1 4n + 1 has to be an integer. Therefore 4 n 4n must be an integer. For that to happen, n n has to be at least a rational.

Being an odd integer, means that 4 n + 1 4n + 1 = 2 k + 1 2k + 1 , where k Z k \in \mathbb Z . Isolating n n gives us n = k 2 n = \frac{k}{2} .

Now, because 0 n 100 0 \leq n \leq 100 , 0 k 200 0 \leq k \leq 200 . Counting every integer that is between 0 and 200, reveals 201 possible k k s.

Therefore, there are 201 possibles values of n n in [ 0 , 100 ] [0, 100] that holds the property.

Christian Barrera
Dec 23, 2013

An easy approach is checking from the end results (4n + 1) The range of results will be from 1 (4 * 0 + 1) and 401 (4 * 100 + 1)

We can then rephrase the problem to: How many odd integers are there between 1 and 401 inclusive. This can easily be calculated with Round down[(n+1)/2].

The number of odd numbers from 1 to 401 is Rounddown[ (401+1)/2 = 201

Please do note that this is possible because we have the values of n as any real number between 0 and 100 inclusive. If we were restricted to integers we would be unable to get values such as 3, 7, 11, 15... which are the results of n being non-integer (1/2, 3/2, 5/2...)

Christian Barrera - 7 years, 5 months ago
Amlan Mishra
Feb 23, 2014

The values of n satisfying the condition are

0/4, 2/4, 4/4, 6/4, ......., 400/4

So there are 400/2 + 1 no. of values of n satisfying the condition.

So there are 201 possible values for n.

Michael Tang
Dec 21, 2013

All odd integers are in the form 2 k + 1 , 2k+1, for some integer k . k. So, if 4 n + 1 4n+1 is an odd integer, there must exist some integer k 0 k_0 such that 4 n + 1 = 2 k 0 + 1. 4n+1=2k_0+1. Solving for n , n, we get n = k 0 2 . n = \dfrac{k_0}{2}. Thus, the values of n n we want are exactly those that are one-half of an integer. In the given range, there are 201 \boxed{201} possible values of n , n, specifically 0 2 , 1 2 , 2 2 , , 200 2 . \dfrac{0}{2}, \dfrac{1}{2}, \dfrac{2}{2}, \ldots, \dfrac{200}{2}. \square

If 4 4 is multiplied by a number of the form 2 k 1 4 \frac {2k - 1}{4} , you'll get 2 k 1 + 1 = 2 k 2k - 1 + 1 = 2k , which is not odd. That means that the numerator must be even, and then the fraction will be expressed with 2 2 as the denominator. 100 = 200 2 100 = \frac {200}{2} . That means that for the numerator, we can choose all the numbers from 0 0 to 200 200 . In this interval there are 201 201 numbers. Hence, the answer is 201 \boxed {201} .

Every integer from 0 to 100 works, as 4n is an even number and + 1 makes it an odd integer. Furthermore, every half also works from 0.5 to 99.5 , which is another 100 values. Thus , there are 101 + 100 values that make it odd.

Vishnu Kulkarni - 7 years, 5 months ago

Yes. Those are the cases in which the numerator is divisible by 2 2 .

Diego E. Nazario Ojeda - 7 years, 5 months ago
Asher Abraham
Mar 27, 2020

Let n be of the form n= k 4 \frac{k}{4}
The range of n is [0,100] so the range of k is [0,400]
This implies 4n+1=k+1
So for all even values of k, 4n+1 is odd.
So the possible values of k are 0,2,4,......,398,400.
Therefore there are 201 values of k possible.
Therefore there are 201 values of n possible.





Emanuele Prati
Apr 19, 2019

If 4 n + 1 4n+1 is an integer, 4 n 4n is an integer too, so n n must be equal to a rational number p q \frac{p}{q} where q 4 q \mid 4 ( q q divides 4 4 ). If q = 4 q=4 , 4 n + 1 = p + 1 4n+1=p+1 so p p must be even, therefore simplifying p p with q q , the fraction will have denominator 2 2 . If the fraction has denominator 2 2 , 4 n + 1 = 2 p + 1 4n+1=2p+1 , so it will be odd for each p p integer; if 0 n 100 0 \le n \le 100 and n = p 2 n=\frac{p}{2} , 0 p 200 0 \le p \le 200 , thus p p can assume 201 \boxed{201} integer values. In this are also included integer values of n n , when p p is even.

K T
Mar 15, 2019

Note that n is a real number in the range [0,100]. 4n+1 odd <=> 4n even <=> 2n is an integer. So n can be 0, 1/2, 1, 3/2, ..., 100, these are 201 values.

Auro Light
Sep 12, 2017

4n+1, is an odd integer, means 4n is even,

which in turn means 4n/2 = 2n is any

integer. This further means n can be any

integer k, from 0 to 100 (101 possible

values), as well as k/2 for each k.

So k together with k/2 can have 201

possible values for n (note for k =0, k = k/2).

Vikash Sah
Apr 7, 2014

4n+1 is itselft odd with any +ve integer value. so, from 0 to 100 are solutions and 1/2,3/2, 5/2, ......, 199/2 satisfy. so there are all 201 solutions............

Adwait Sharma
Mar 5, 2014

All the values of n such that n=x/4 where x is even satisfies the condition.

Since n is less than or equal to 100, x should be less than or equal to 400.

So the numbers 0/4, 2/4, 4/4, 6/4 ......400/4 satisfies the condition and the number of terms can be easily found using the concept of arithmetic progression

Vaibhav Agarwal
Mar 5, 2014

for 4n +1 to be an odd integer, 4n has to be an even integer.

For n=an integer, all 101 values from [0,100] will satisfy, moreover even the values of the form z+ 0.5, where z is an integer, will be 4z+2 when multiplied by four and thus will be even.Thus we have another 1 set of values, satisfying us. Thus the total no. of values will be 100+ 101=201

0 to 10.5 containing 22 numbers at an interval of 0.5, 11 to 20.5, 21 to 30.5 ... so on till 81 to 90.5, 91 to 100 so basically it is 22+20 *8 times and 19 for the last number line. is 201

Nicholas Tomlin
Dec 22, 2013

If 4 n + 1 4n+1 must be an odd integer, then 4 n 4n must be an even integer.

For n [ 0 , 100 ] n\in [0,100] , 4 n 4n ranges from 0 0 to 400 400 , inclusive. There are 201 201 even numbers in the range [ 0 , 400 ] [0,400] , so there must be 201 \boxed{201} corresponding values of n n between 0 0 and 100 100 , inclusive.

Desmond Kan
Dec 22, 2013

First observe that 1 is an odd integer where n is chosen to be 0. Next observe that 401 is an odd integer where n is chosen to be 100. Every odd integer between 1 and 401 can be expressed in the form 4n+1 where n is a real value in (0, 100). Therefore this question is equivalent to counting the number of odd integers in [1, 401], which is simply just (401+1)/2 = 201.

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