Product of numbers

The value of ( 1 + 1 2 ) ( 1 + 1 3 ) ( 1 + 1 100 ) \left(1 + \frac{1}{2}\right) \left( 1 + \frac{1}{3} \right) \cdots \left( 1 + \frac{1}{100}\right) can be expressed as a b \frac{a}{b} where a a and b b are coprime positive integers. What is the value of a + b ? a +b?


The answer is 103.

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18 solutions

升泽 林
Dec 17, 2013

( 1 + 1 2 ) ( 1 + 1 3 ) . . . ( 1 + 1 100 ) (1+\frac{1}{2})(1+\frac{1}{3})...(1+\frac{1}{100})

= 3 2 × 4 3 . . . × 101 100 =\frac{3}{2}\times\frac{4}{3}...\times\frac{101}{100}

= 101 2 =\frac{101}{2}

a + b = 103 a+b=\boxed{103}

I don't get it~ (_ _")

Mohammad Hafizie Dianel - 7 years, 5 months ago

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Step by step:

Let n n be any positive number, 1 + 1 n = n + 1 n 1+\frac{1}{n}=\frac{n+1}{n}

So, 1 + 1 2 = 2 + 1 2 = 3 2 1+\frac{1}{2}=\frac{2+1}{2}=\frac{3}{2}

1 + 1 3 = 4 3 1+\frac{1}{3}=\frac{4}{3}

1 + 1 4 = 5 4 1+\frac{1}{4}=\frac{5}{4} ......And so on!

Therefore, ( 1 + 1 2 ) ( 1 + 1 3 ) ( 1 + 1 4 . . . ( 1 + 1 99 ) ( 1 + 1 100 ) (1+\frac{1}{2})(1+\frac{1}{3})(1+\frac{1}{4}...(1+\frac{1}{99})(1+\frac{1}{100})

= 3 2 × 4 3 × 5 4 . . . × 100 99 × 101 100 \frac{3}{2}\times\frac{4}{3}\times\frac{5}{4}...\times\frac{100}{99}\times\frac{101}{100}

Now, it is the MOST IMPORTANT step:

First, note that if a fraction times another fraction, and if the numerator of one of the fractions is equal to the denominator of another fraction, we can simplify them to 1 1 .

Example: a b × c a = 1 × c b × 1 = c b \frac{a}{b}\times\frac{c}{a}=\frac{1\times c}{b\times1}=\frac{c}{b}

Now let's look our question.

Use the same way of simplifying the fractions, we get the answer = 101 2 =\frac{101}{2}

So the final answer = 101 + 2 = 103 =101+2=\boxed{103} :D

Can you got it?

A Former Brilliant Member - 7 years, 5 months ago

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I get it :D I Think I have to review the basics :)

Vincent Fernandez - 7 years, 5 months ago

thank you...

divya lakshmi - 7 years, 5 months ago

thanks your way is amazing

Ayush Sharma - 7 years, 5 months ago
Jordan Osborn
Aug 25, 2013

Note we can rewrite as ( 3 2 ) ( 4 3 ) . . . ( 100 99 ) ( 101 100 ) (\frac{3}{2})(\frac{4}{3})...(\frac{100}{99})(\frac{101}{100}) All terms will cancel except the first denominator and the last numerator so it will simplify to 101 2 \frac{101}{2} Hence solution is 101 + 2 = 103 101+2=103

103

Imààn Boù - 7 years, 9 months ago
Ho Wei Haw
Aug 26, 2013

Firstly, 1 + 1 n = n + 1 n 1+\frac{1}{n}=\frac{n+1}{n}

Then, we have

n = 2 N n + 1 n = 3 2 × 4 3 × . . . × N N 1 × N + 1 N = N + 1 2 \prod_{n=2}^{N}\frac{n+1}{n}=\frac{3}{2}\times\frac{4}{3}\times...\times\frac{N}{N-1}\times\frac{N+1}{N}=\frac{N+1}{2}

So, with the given expression, we can rewrite it into as such

n = 2 100 n + 1 n \prod_{n=2}^{100}\frac{n+1}{n}

which is equivalent to

100 + 1 2 = 101 2 \frac{100+1}{2}=\frac{101}{2}

a + b = 103 \therefore a+b=103

Ajay Maity
Dec 18, 2013

( 1 + 1 2 ) ( 1 + 1 3 ) ( 1 + 1 4 ) . . . . . . . ( 1 + 1 99 ) ( 1 + 1 100 ) (1 + \frac{1}{2})(1 + \frac{1}{3})(1 + \frac{1}{4}).......(1 + \frac{1}{99})(1 + \frac{1}{100})

= ( 3 2 ) ( 4 3 ) ( 5 4 ) . . . . . ( 100 99 ) ( 101 100 ) = (\frac{3}{2})(\frac{4}{3})(\frac{5}{4}).....(\frac{100}{99})(\frac{101}{100})

Everything gets cancelled, only the first denominator term and the last numerator term remain.

Therefore the result a b = 101 2 \frac{a}{b} = \frac{101}{2}

So, a + b = 101 + 2 = 103 a + b = 101 + 2 = \boxed{103}

That's the answer!

got it... best answer...

Tushar Dharade - 7 years, 5 months ago

best answer

Aniya Malik - 7 years, 5 months ago

I prefer it

Mohammad Chhiddikur Rahman - 7 years, 4 months ago

thanx

mandeep manchanda - 7 years, 4 months ago
Tamojit Maiti
Aug 26, 2013

Firstly we try to observe and identify any pattern that may exist between the terms. On inspection, we find that t n t_n = n + 1 n + 2 \frac{n+1}{n+2} .

Now on evaluating the product for the given sequence, we notice that numerator of the t n t_n term equals the denominator of the t n 1 t_{n-1} term; and hence cancel out.Hence the product evaluates to the denominator as the denominator of the first term ie 2, and the numerator as the numerator of the last term, ie 101.

( 1 + 1 2 ) × ( 1 + 1 3 ) × ( 1 + 1 4 ) × × ( 1 + 1 100 ) ({1+ \frac{1}{2}}) \times ({1 + \frac{1}{3}}) \times ({1 + \frac{1}{4}}) \times \dots \times ({1 + \frac{1}{100}}) = 3 2 × 4 3 × 5 4 × × 101 100 \frac{3}{2} \times \frac{4}{3} \times \frac{5}{4} \times \dots \times \frac{101}{100} = 101 2 \frac {101}{2}

Can you clarify what is t n t_n and why t n = n + 1 n + 2 t_n = \frac{n+1}{n+2} ?

Calvin Lin Staff - 7 years, 9 months ago
Mulubrhan Desta
Aug 25, 2013

(1+1/2)(1+1/3)...(1+1/100) =(3/2)(4/3)(5/4)...(100/99)(101/100). Now you can easily see the the numerator of the first term can be cancelled with the denominator of the next term and goes on. Finally, we have the numerator of the last term(101) and denominator of the first term(2). Hence, 101/2. Therefore, a+b=103.

The general form of each factor is 1 + 1 n = 1 + n n 1 + \frac{1}{n} = \frac {1 + n}{n} . So we may infer that the product ( 1 + 1 2 ) ( 1 + 1 3 ) . . . ( 1 + 1 100 ) = ( 3 2 ) ( 4 3 ) . . . ( 101 100 ) = ( 1 + \frac{1}{2})( 1 + \frac{1}{3})...( 1 + \frac{1}{100}) = (\frac{3}{2})(\frac{4}{3})...(\frac{101}{100}) = = ( 2 2 ) ( 3 2 ) ( 4 3 ) . . . ( 101 100 ) = 101 ! 2 × 100 ! = = (\frac {2}{2})(\frac{3}{2})(\frac{4}{3})...(\frac{101}{100}) = \frac{101!}{2 \times 100!} = = 101 2 = a b = \frac{101}{2} = \frac {a}{b} .

Hence a + b = 101 + 2 = 103 a + b = 101 + 2 = 103

3/2 4/3 5/4 6/5 7/6 ........99/98 100/99*101/100 all the denominators get cancelled with the numerator of next value leaving 101/2 in the end

Abhishek Sinha - 7 years, 9 months ago
Abel Chen
Aug 25, 2013

Rewriting each term as n + 1 n \frac{n+1}{n} we can see that the numerator of any term is the same as the denominator of the next term, so they cancel each other out. This leaves only the denominator of the first term and the numerator of the last term, which results in the fraction 101 2 \frac{101}{2} .

101 + 2 = 103 101+2=103

Abhinav Ankur
Mar 22, 2014

The answer is 103

3/2x4/3x............101/100=101/2

Gautam Shenoy
Dec 19, 2013

= 3 2 4 3 5 4 . . . 101 100 = \frac{3}{2}\frac{4}{3}\frac{5}{4} ... \frac{101}{100}

Hùng Minh
Dec 18, 2013

(1 + 1/2)(1+1/3)(1+1/4).....(1+1/96)(1+1/97)(1 + 1/98)(1 + 1/99)(1 + 1/100) = 2 x 3/2 x 4/3 x ....49/48 x 50/49 x 101/100. There is 49 pair number. So 2 x 3/2 x 4/3 x ....49/48 x 50/49 x 101/100 = 50 x 101/100 = 101 / 2 <=> a = 101, b = 2 <=> a + b = 103.

We operate this multiplication of fraction to be \frac{3}{2}.\frac{4}{3}.\frac{5}{4}.\frac{6}{5}.....\frac{101}{100} Simplify the denominator of a factor with the nominator of the adjacent factor. And then we get a fraction \frac{101}{2}. And so the answer is 103

Note that 1 + 1 x + k = x + k + 1 x + k 1 + \frac{1}{x+k} = \frac{x+k+1}{x+k} , and also that x + 1 x × x + 2 x + 1 = x + 2 x \frac{x+1}{x} \times \frac{x+2}{x+1} = \frac{x+2}{x} .

This product telescopes, and we can note that by multiplying the first terms: 3 2 × 4 3 × 5 4 × × 101 100 = 101 2 \frac{3}{2} \times \frac{4}{3} \times \frac{5}{4} \times \cdots \times\frac{101}{100} = \frac{101}{2} . Thus, a + b = 103. a +b = 103.

Noel Quirol
Aug 28, 2013

(1+1/2)(1+1/3)(1+1/4)...(1+1/100) => (3/2)(4/3)(5/4)... (101/100)

We can clearly see that EVERYTHING CAN BE CANCELLED OUT , Except the DENOMINATOR of the FIRST(3/2) and the NUMERATOR of the LAST(101/100)

So we have; (1/2)(101/1) => 101/2

a+b = 101+2 => 103.

Done :))

Leonardo Cidrão
Aug 27, 2013

Rewrite the expression: Productory of (1+ 1/n) from n=2 to 100. After this, you may realize the numbers above will be divided by the numbers above (in the fractions). Therefore we get: 101/2; where a=101;b=2 ----> a+b=101+2=103

Alex Benfield
Aug 26, 2013

(1+1/2)(1+1/3)... can be written as: 3/2 * 4/3 * 5/4 ...... 101/100. We are simply just writing out each bracket as a fraction. We can now see that most of the integers in the numerator are also in the denominator. We can cancel out all these numbers. In fact all numbers cancel out apart from 101 in the numerator and 2 in the denominator. Thus you get 101/2. And so 101+2=103

Frank Tieskens
Aug 26, 2013

this can be rewritten as 3/2 * 4/3 * 5/4 * ... * 101/100

All the denominators and dividers cancel out, since a/b*b/c=a/c if b and c are real and unequal to zero, except for 101 and 2, so a/b = 101/2

101 and 2 are coprime positive integers and cannot be further simplified, so a=101 and b=2, hence a+b=103

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