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# C2 geometric sequences- least value of n watch

1. " Find the least value of n such that the sum 3+6+12+24+... to n terms would first exceed 1.5 million"

This is my working attached and my answer is slightly different to that of the book's. (The book's answer is my last line of working). The answer I got to was 19.93. Can someone tell me where I went wrong?
2. (Original post by jessyjellytot14)
Can someone tell me where I went wrong?
there should be a fullstop after million
there should be a fullstop after million
Will I get the right answer if I put a full stop after million?
Spoiler:
Show
There should be a space between 'full' and 'stop'.
4. it is a GP with a = 3 & r = 2

Sn = {a ( 1 - r n )} / ( 1 - r ) > 1500000

put in the values for a, r & find n. you may need logarithms ?
5. (Original post by jessyjellytot14)
" Find the least value of n such that the sum 3+6+12+24+... to n terms would first exceed 1.5 million"

This is my working attached and my answer is slightly different to that of the book's. (The book's answer is my last line of working). The answer I got to was 19.93. Can someone tell me where I went wrong?
The sum, not the term
6. am I too late?
7. (Original post by TeeEm)
am I too late?
yup , i was 22 mins too late ;(
8. (Original post by Naruke)
The sum, not the term
(Original post by the bear)
it is a GP with a = 3 & r = 2

Sn = {a ( 1 - r n )} / ( 1 - r ) > 1500000

put in the values for a, r & find n. you may need logarithms ?
Ohhh okay it makes sense now

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