A-Level Maths: D5-07 Geometric Sequences: Examples of Finding the Sum of Geometric Series

[@jackkselby]

sorting me out for my mocks, best maths channel

[@Masowe.]

7:07 where did the 16go? i am really confused

[@TLMaths]

I can't see a 16 disappearing?

[@Masowe.]

@@TLMaths oh year thank you, now i get that it was used before applying log

[@Masowe.]

now i get what was happening, thank you for the playlist, i wish i knew about it earlier

[@lukeollerhead9748]

The sigma notation has a slightly different convention in the Edexcel textbook. It's always n=0 on the bottom and n-1 on the top. Have you seen this before? So for the first 5 terms of a geometric series, you would get n=0 on the bottom and 4 on the top if that makes sense...

[@samv2907]

For question 2, can you use natural logs as well??

[@TLMaths]

Yep

[@olivergilkes8804]

should you leave your answers as decimals or fractions?

[@TLMaths]

I would aim to keep answers as exact, whether that be decimals or fractions. I will tend to use fractions

[@Doodo981]

For number 6, why is n=3 I don’t understand why you have to add a 1 to 8-6

[@TLMaths]

Because there are 3 terms: n=6, n=7, n=8

[@scottwall157]

Q2. Where does the 2187/128 come from? 136.6875/8 = 17.08

[@TLMaths]

On an up to date scientific calculator like the Casio Classwiz, 136.6875/8 = 2187/128

[@ghsjgsjg53chjdkhjydhdkhfmh74]

Thank you!!

[@jackhatton2330]

Hi Jack, I am following an example on the Edexcel Pure Maths Year 2 book (3.4 - Example 13) I am struggling to follow one particular part of the example Line 1: 1024(-1/2)^n-1 = 1 Line 2: (-2)^n-1 = 1024 I can't work out how to get from Line 1 to Line 2! Any help appreciated.

[@TLMaths]

1024(-1/2)^(n-1) = 1 (-1/2)^(n-1) = 1/1024 1/(-1/2)^(n-1) = 1024 (1/(-1/2))^(n-1) = 1024 (-2)^(n-1) = 1024

[@ibrahimmiah562]

Do you get given these formulas in the exam?

[@ibrahimmiah562]

For OCR...

[@TLMaths]

For sequences, you get the sum to n terms for both arithmetic and geometric (and the sum to infinity for geometric). However, you do not get the nth term formulae.

[@tni3920]

what happens when the common ratio is negative wouldn't it give you a error

[@TLMaths]

My example in number 3 has a negative common ratio? I'm not sure what error you're implying?