Answer Within A Minute III

Algebra Level 1

1 3 × 3 5 × 5 7 × 7 9 × 9 11 × × 47 49 \frac{1}{3} \times \frac{3}{5} \times \frac{5}{7} \times \frac{7}{9} \times \frac{9}{11} \times \cdots \times \frac{47}{49}

If the value of the expression above equals a b \frac ab for coprime positive integers a a and b b , find the value of a + b a+b .


The answer is 50.

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16 solutions

Wenn Chuaan Lim
Apr 2, 2014

Simplify the 1st fraction denominator with the 2nd fraction numerator for example 1 3 \frac{1}{3} with 3 5 \frac{3}{5} to become 1 5 \frac{1}{5} and do it continuously until the last. The final answer will be 1 49 = a b \frac{1}{49} = \frac{a}{b} . Therefore, a+b = 1+49 = 50

I got it in 2 sec

Sai Ram - 5 years, 9 months ago
敬全 钟
Apr 2, 2014

The given expression immediately simplifies to 1 49 \frac{1}{49} , so a+b=50.

You may consider this problem: ( 1000 x 1000 + 999 x 999 + 998 x 998 + . . . + x + 1 ) z 30 = 1 (1000x^{1000}+999x^{999}+998x^{998}+...+x+1)^{z-30}=1 Find z

敬全 钟 - 7 years, 2 months ago

Log in to reply

30

Rishabh Jain - 7 years, 2 months ago

30 . n^0 as such that n isn't equal to 0 is equal to 1.

Lance Fernando - 5 years, 10 months ago

Z can be 30 or any integer if the given polynomial is equal to 1

Department 8 - 5 years, 9 months ago

Write a comment or ask a question...i don't clear. please again solved me. Different rules.

Farid Khan - 5 years, 9 months ago
Samin Sharar
May 31, 2015

We can easily see that denominator of the 1st fraction and numerator of the second fraction are same so they will omit each other and in the last fraction since there is no other fractions numerator to omit its denominator so it will remain and the numerator will be omitted by the previous fraction denominator that is 1/49 will remain that is a=1 and b=19.So a+b=50

Aparna Kalbande
Sep 25, 2015

If look carefully at problem then we get that denominater and numerator of other gets cancelled ans. 1/49 .49+1=50

Hashim Arain
Sep 22, 2015

All the values are canceled from each other in the end we have remain just 1 and 49 so a+b like 1+49=50 so simple :)

Bisma Joyosumarto
Sep 12, 2015

Let's answer by seeing the first five fractions.

1 3 3 5 5 7 7 9 9 11 47 49 \frac {1}{3} * \frac {3}{5} * \frac {5}{7} * \frac {7}{9} * \frac {9}{11} * \ldots * \frac {47}{49}

Do you see how you can cross-multiply ? Look again.

1 3 3 5 = 1 3 3 5 = 1 5 \frac {1}{3} * \frac {3}{5} = \frac {1 * 3}{3 * 5} = \frac {1}{5}

With 1 5 \frac {1}{5} , we can cross-multiply again with 5 7 \frac {5}{7} ...

1 5 5 7 = 1 5 5 7 = 1 7 \frac {1}{5} * \frac {5}{7} = \frac {1 * 5}{5 * 7} = \frac {1}{7}

Do you see a pattern?

1 7 7 9 = 1 7 7 9 = 1 9 \frac {1}{7} * \frac {7}{9} = \frac {1 * 7}{7 * 9} = \frac {1}{9}

The numerator is always 1, while the denominator will be the denominator of the last fraction we just cross-multiplied.

1 9 9 11 = 1 9 9 11 = 1 11 \frac {1}{9} * \frac {9}{11} = \frac {1 * 9}{9 * 11} = \frac{1}{11}

Therefore, with the last fraction as 47 49 \frac {47}{49} , multiplying everything manually will get you 1 49 \frac {1}{49} , don't you think?

1 47 47 49 = 1 47 47 49 = 1 49 \frac {1}{47} * \frac {47}{49} = \frac {1 * 47}{47 * 49} = \frac {1}{49}

This problem then tells us to make that fraction equal to a b \frac {a}{b} ...

1 49 = a b \frac {1}{49} = \frac {a}{b}

a = 1 a = 1

b = 49 b = 49

... then tells us to add them together.

a + b = 1 + 49 = 50 a + b = 1 + 49 = \boxed{50}

Done!

Kushagra Pareek
Sep 11, 2015

Its too easy, i got in 2 sec

Tushar Kaushik
Sep 9, 2015

Cancel them diagonally 1&49 will be left add them up U lk get 50

Just cancel out every same numerator and denominator until 1/49 is left. Add both numbers and you'll get 50

Liam Bennett
Sep 3, 2015

On the surface these types of questions always seem tricky for many of the "mathematically challenged people" (many of whom I don't suppose are reading this right now). What I would honestly say to you is simply to identify a pattern. I have seen many intelligent people try to overthink a problem like this one and get it completely wrong; I see this happening a lot. You must realize that these problems aren't trying to trick you. So the next time you look at one of these questions try not to misatribute what the question is really trying to ask. Hope this comment helps in some way to the people who didn't get the right answer.

Bharath A
Sep 1, 2015

Wooooooow easy one was that ..just a simple logic..the 1 St term numerator and the last trems denomentaor does not get cancelled their by u get the a/B fraction sub their value in a+B=1+49=50

Shekhar Chaudhary
Aug 29, 2015

I got in 0.1sec 1/49 .1+49=50

Airene Raguine
Aug 29, 2015

By inspection, you can derive 1/49 = a/b, so a+b=50.

Lance Fernando
Aug 1, 2015

Notice the pattern that it cancels out diagonally upward, so it leaves out 1/49. When added together, it becomes 50.

a+b=1+49=50

Harshad Kale
Apr 19, 2014

ALL NOS WILL cut out just 49 will be in the denominator hence 49+1=50

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