Need help for infinite sigma question (1 Viewer)

[BlueGas]

I need help with question 19 a). Here's the question:

 

[VBN2470]

Use limiting sum formula, answer should be

 

[BlueGas]

Use limiting sum formula, answer should be

The answer is 6 2/3, but I have no clue on working out this question.

 

[VBN2470]

Sorry, I edited my original post, didn't realise it said '1/4' not '1/2'. Do you understand it now? You just need to use the limiting sum formula.

 

[BlueGas]

Sorry, I edited my original post, didn't realise it said '1/4' not '1/2'. Do you understand it now? You just need to use the limiting sum formula.

I get how a = 5 after you sub n = 1, but how do you know that r = 1/4?

 

[VBN2470]

The 'r' refers to the ratio of the geometric series, in this case each subsequent term is being multiplied by a factor of 1/4, hence the ratio r = 1/4.

 

[VBN2470]

Write out the series as you normally would, and see if you can identify that the series is in fact geometric with r = 1/4.