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# P1 - Sequences and Series. watch

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1. I have a couple of questions that I'm stuck on, wondering if anyone can help....

1.
10
∑ 2r - 3
r=1

i have to write down the first 4 terms.....

2. find the sum of the following geometric series

50 + 40 + 32 + .......

i worked out r = 0.8, but i don't know how to work out how many terms there are to find the sum...

and finally...

3. in a geometric series sequence Un, U2 = 4 and U4 = 16/9

find the two possible values of its common ration r and first term a.

Any help with any of these would be great..... thanks.
2. 3. in a geometric series sequence Un, U2 = 4 and U4 = 16/9

find the two possible values of its common ration r and first term a.

Any help with any of these would be great..... thanks.[/QUOTE]

aq=4
aq3=16/9
aq3/aq=16/9/4
q(common difference)= +2/3,-2/3
aq=4, a=4/q, a=6 or a= -6
3. 2. find the sum of the following geometric series

50 + 40 + 32 + .......

I have forgotten the formula, but you have to do a sum to infinity
4. (Original post by ainat87)
1.
10
∑ 2r - 3
r=1

i have to write down the first 4 terms.
-1, 1, 3, 5
5. (Original post by ainat87)
I have a couple of questions that I'm stuck on, wondering if anyone can help....

1.
10
∑ 2r - 3
r=1

i have to write down the first 4 terms.....

2. find the sum of the following geometric series

50 + 40 + 32 + .......

i worked out r = 0.8, but i don't know how to work out how many terms there are to find the sum...

and finally...

3. in a geometric series sequence Un, U2 = 4 and U4 = 16/9

find the two possible values of its common ration r and first term a.

Any help with any of these would be great..... thanks.
2. if r<1 then sum to infinity =a/(1-r)=50/0.2=250
6. 10
∑ 2r - 3
r=1

Doesn't the 10 above mean work to the first 10 terms?

Anyway. put r equal to 1, 2, 3 and 4 to get:

2*1-3, 2*2-3, 2*3-3, 2*4-3

which is
-1, 1, 3, 5
7. (Original post by mik1a)
10
∑ 2r - 3
r=1

Doesn't the 10 above mean work to the first 10 terms?

Anyway. put r equal to 1, 2, 3 and 4 to get:

2*1-3, 2*2-3, 2*3-3, 2*4-3

which is
-1, 1, 3, 5
no it means sum of all (2r-3), from when r=1 to r=10 which i suppose it wot u said..
8. wow. thanks you guys.

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