Question Details

Evaluate the value of the following summation series:

135+163+199++1255=?

Options

A

885

B

685

C

485

D

1285

Show Answer

Correct Answer :

Option B

685

Solution :

The correct answer is 685.

To evaluate the value of the given summation series, let us first analyze the structure of each denominator:

35=5×7

63=7×9

99=9×11

255=15×17

Notice that every term in the series is of the general form 1n(n+2), where the two factors in the denominator have a common difference of 2.

We can decompose any fraction of this form into partial fractions using the following standard identity:

1n(n+2)=12(1n-1n+2)

Using this identity, we rewrite each term of the series individually:

135=15×7=12(15-17)

163=17×9=12(17-19)

199=19×11=12(19-111)

1255=115×17=12(115-117)

Now, we sum all the terms together and factor out the common multiplier of 12:

Sum=12[(15-17)+(17-19)+(19-111)++(115-117)]

This is a telescoping series, which means all intermediate positive and negative terms cancel each other out:

-17+17=0, -19+19=0, , -115+115=0

After these cancellations, only the first term 15 and the last term 117 remain inside the brackets:

Sum=12(15-117)

Find the common denominator of 5 and 17, which is 85:

15-117=17-585=1285

Finally, multiply this result by 12:

Sum=12×1285=685

Thus, the final evaluated value of the summation series is 685.

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