Warm up problem 7: telescoping series easy-medium

Algebra Level 3

If the value of

1 3 + 1 15 + 1 35 + . . . 1 483 \frac{1}{3}+\frac{1}{15}+\frac{1}{35}+...\frac{1}{483}

Can be expressed as a b \frac{a}{b} where a,b are positive co prime integers, find a+b.


The answer is 34.

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1 solution

Trevor Arashiro
Sep 24, 2014

Finalized my solution. To understand this solution, it is helpful if you read this inorder to understand the Heaviside function. However, if you have trouble understanding this article or my solution, feel free to ask any question you may have. I'd be more than happy to answer it as telescoping functions are great but can get quite tricky.

By Heaviside function. 1 a ( a + 2 ) = 1 2 ( 1 a 1 a + 2 ) \dfrac{1}{a(a+2)}=\dfrac{1}{2}\left(\dfrac{1}{a}-\dfrac{1}{a+2}\right)

1 3 + 1 15 + 1 35 + . . . 1 483 \dfrac{1}{3}+\dfrac{1}{15}+\dfrac{1}{35}+...\dfrac{1}{483}

1 1 × 3 + 1 3 × 5 + 1 5 × 7 + . . . 1 21 × 23 \dfrac{1}{1 \times 3}+\dfrac{1}{3 \times 5}+\dfrac{1}{5 \times 7}+...\dfrac{1}{21\times 23}

1 2 ( ( 1 1 3 ) + ( 1 3 1 5 ) + ( 1 5 1 7 ) . . . + ( 1 21 1 23 ) ) \dfrac{1}{2}\left( \left( 1-\dfrac{1}{3} \right)+\left(\dfrac{1}{3}-\dfrac{1}{5}\right)+\left(\dfrac{1}{5}-\dfrac{1}{7}\right)...+\left(\dfrac{1}{21}-\dfrac{1}{23}\right) \right)

1 2 ( 1 1 23 ) \dfrac{1}{2}\left( 1-\dfrac{1}{23} \right)

11 23 \dfrac{11}{23}

11 + 23 = 34 \boxed{11+23=34}

nice problem

math man - 6 years, 8 months ago

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Thank you. It's great to have feed back so I know what and what not to post

Trevor Arashiro - 6 years, 8 months ago

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Definitely you post nice ones Trevor! :)

Krishna Ar - 6 years, 8 months ago

I have finalized my solution so it is easier to understand. Please feel free to ask if you have any questions.

Trevor Arashiro - 6 years, 8 months ago

And who else thought that Heaviside was spelled heavy side... I honestly never knew how to spell it until I looked it up for my solution.

Trevor Arashiro - 6 years, 8 months ago

Could have called it Telescoping technique!! Rather than "Heavy side"! Lol! :P

Pranjal Jain - 6 years, 7 months ago

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