Arithmetic and Geometric Progressions

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Infinite terms of a series

Sum of infinite terms

    If , where denotes the sum of the first n terms of a series and = 291, then m is equal to?
    1. 24
    2. 30
    3. 26
    4. 20

 

  • Correct Answer
    Choice B. 30

Detailed Solution

The pattern is as follows : 2, 23, 234...



Substitute m instead of n

We know that
Hence





Solving above equation we get m = -29, 30
M cannot be negative
Hence m = 30


Correct Answer: 30



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With some simple but very powerful ideas, one can cut down on a lot of working when it comes to progressions. For example, anchoring a progression around its middle term can be very useful. Reinforce these ideas with these questions.