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# geometric progession help Watch

1. no

3 * 5^x = 150

5^x = 50

log5^x = log50

xlog5 = log50

x = log50/log5

Can I ask if you are self teaching this material ... if not, there seems to be a lot that you have not understood from lessons
2. (Original post by dongonaeatu)
is the sum to infinity 16.6
as my post said ... NO
3. (Original post by dongonaeatu)
hey man, solve 3*5^x=150 giving it to 4dp

is it; log3*5^x=log150

5^x=log150/log3

5^x=4.560876795 then i dont know how to get the x on its own

Now take log of both sides and remember,
4. (Original post by raheem94)

Now take log of both sides and remember,
oh i see so you get rid of the 3* so its 5^x=50 then do i do

log5^x=log50
5. (Original post by dongonaeatu)
oh i see so you get rid of the 3* so its 5^x=50 then do i do

log5^x=log50
Yes
6. (Original post by TenOfThem)
no

3 * 5^x = 150

5^x = 50

log5^x = log50

xlog5 = log50

x = log50/log5

Can I ask if you are self teaching this material ... if not, there seems to be a lot that you have not understood from lessons
thanks that was helpful, is the answer 2.4307? and no i do AS maths at college
7. (Original post by dongonaeatu)
oh i see so you get rid of the 3* so its 5^x=50 then do i do

log5^x=log50
By the way, you need to learn the log chapter again. Going through your book will be more helpful than asking questions here.

I am quite sure if you learn the chapter well, then you should be able to answer most of these questions.
8. (Original post by dongonaeatu)
thanks that was helpful, is the answer 2.4307? and no i do AS maths at college
Yes
9. (Original post by raheem94)
By the way, you need to learn the log chapter again. Going through your book will be more helpful than asking questions here.

I am quite sure if you learn the chapter well, then you should be able to answer most of these questions.
thanks man i will do that
10. (Original post by raheem94)
Yes
The first term of a geometric progression is 5 and the third term is 10
i. determine the two possible values for the common ratio [2marks]

so a=5 and ar is the second term so ar^2=10

so 5(r)^2=10

r^2=5 so r=square root of 5 so thats 2.236067977 or -2.236067977

is this right
11. (Original post by raheem94)
Yes
sorry that was wrong

i think its: ar^2=10
5(r)^2=10
so then i divide by 5 NOT minus 5 like i did last time becauses it's 5 times

so r^2=2
so r= square root of 2 which equals 1.414213562 or -1.414213562 please tell me this is correct
12. (Original post by dongonaeatu)
sorry that was wrong

i think its: ar^2=10
5(r)^2=10
so then i divide by 5 NOT minus 5 like i did last time becauses it's 5 times

so r^2=2
so r= square root of 2 which equals 1.414213562 or -1.414213562 please tell me this is correct
It is correct
13. (Original post by raheem94)
It is correct
yay!!

ii. Using the largest of these two values for the common ratio find the first term to exceed 5000. [3 marks]

so i am using the positive 1.414213562 but i really dont know how to tackle this type of question
14. (Original post by dongonaeatu)
yay!!

ii. Using the largest of these two values for the common ratio find the first term to exceed 5000. [3 marks]

so i am using the positive 1.414213562 but i really dont know how to tackle this type of question
You know that nth term

So here,

Find the value of 'n'.
15. (Original post by raheem94)
You know that nth term

So here,

Find the value of 'n'.
so 5000= 5(1.414213562)^n-1

5000=7.07106701^n-1

how do i get n
16. (Original post by dongonaeatu)
so 5000= 5(1.414213562)^n-1

5000=7.07106701^n-1

how do i get n
Take the log of both sides

Also to make it easier, keep it in root form

As in, instead of 1.414213562, use
17. (Original post by Joshmeid)
Take the log of both sides
this isnt a log question its geometric progression
18. (Original post by dongonaeatu)
so 5000= 5(1.414213562)^n-1

5000=7.07106701^n-1

how do i get n
You really need to go through the chapter again.

19. (Original post by raheem94)
You really need to go through the chapter again.

thats insane, and i never knew logs were used in geometric progressions; how did you know to use logs
20. (Original post by dongonaeatu)
thats insane, and i never knew logs were used in geometric progressions; how did you know to use logs
The only way to solve this was to use logs, as said before, you need to go through your book again.

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