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# geometric progession help

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1. first two terms of geometric progression are 10 and 4
i. find the sum of the first 15 terms
ii. find the sum to infinity in exact form

so sn=a(r^n-1)/r-1 a=10 n=15 and r=0.4

s15=10(0.4^14)/0.4-1= -4.473924267*10^-2 is this right?
2. No it's not.

You seem to be missing a -1 in the numerator and it's to the power 15.
3. it is
4. (Original post by dongonaeatu)
first two terms of geometric progression are 10 and 4
i. find the sum of the first 15 terms
ii. find the sum to infinity in exact form

so sn=a(r^n-1)/r-1 a=10 n=15 and r=0.4

s15=10(0.4^14)/0.4-1= -4.473924267*10^-2 is this right?
No.

and the sum of infinity for |r|<1

5. (Original post by dongonaeatu)
first two terms of geometric progression are 10 and 4
i. find the sum of the first 15 terms
ii. find the sum to infinity in exact form

so sn=a(r^n-1)/r-1 a=10 n=15 and r=0.4

s15=10(0.4^14)/0.4-1= -4.473924267*10^-2 is this right?
The formula for sum is,

So inputting the values gives,
6. (Original post by ztibor)
No.

and the sum of infinity for |r|<1

i got 1.666638771 but i guess thats wrong too as its the sum of the first 15 terms and the first term is 10
7. (Original post by raheem94)
The formula for sum is,

So inputting the values gives,
raheem you tank

is the answer 16.66664877
8. (Original post by ztibor)
No.

and the sum of infinity for |r|<1

thats wrong the a should be above the division line
9. (Original post by dongonaeatu)
raheem you tank

is the answer 16.66664877
Yes
10. (Original post by dongonaeatu)
thats wrong the a should be above the division line
It doesn't matters, ztibor is correct.
11. (Original post by raheem94)
It doesn't matters, ztibor is correct.
ah okay. What is the formula for the sum to infinity
12. (Original post by dongonaeatu)
ah okay. What is the formula for the sum to infinity
ztibor has given you that
13. (Original post by ztibor)
No.

and the sum of infinity for |r|<1

it says find the sum to infinity in exact form so.

sum to infinity= 10/1-0.4= 16.6?
14. (Original post by TenOfThem)
ztibor has given you that
is the sum to infinity 16.6
15. 10/0.6 = 100/6 = 50/3

you seem to have rounded so that would not be exact form
16. (Original post by raheem94)
It doesn't matters, ztibor is correct.
is the sum to infinity 16.6
17. (Original post by dongonaeatu)
is the sum to infinity 16.6
Yes, it is 16.7(to 3s.f.).
18. (Original post by TenOfThem)
10/0.6 = 100/6 = 50/3

you seem to have rounded so that would not be exact form
yeah because it had lots of 6's in it lol. can i just leave it as 50/3 then?
19. (Original post by dongonaeatu)
yeah because it had lots of 6's in it lol. can i just leave it as 50/3 then?
The question asks for exact answer hence you MUST write it as
20. (Original post by raheem94)
The question asks for exact answer hence you MUST write it as
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

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