Series question !!! (1 Viewer)

[trungduong12]

Find value of k for which 81920 is the 8th term of the series 5+k+80+....

I do it this way:
K/5 = 80/K
K^2 = 400
K = 20

The answer is k = 20

But i dont get why the question says 81920 is the 8th term??

HELPPPPPP !!!




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[aDimitri]

because otherwise it could be an arithmetic series, and k would be (80+5)/2

 

[trungduong12]

It cant be AP cuz when i sub back in it doesnt work

0.0

I know that we should use what the question given to figure AP or GP but pls somehow show me how


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[Squar3root]

Well the question needs to tell you whether it is a AP or GP. By judging from the exponential increase, I would say it is GP

 

[Squar3root]

But then again notsureifsrs.

OP; you've only done one "part" of the question, finding k. You've made no attempt in finding the 8th term

 

[RealiseNothing]

It tells you what the 8th term is so you can figure out whether or not it is AP or GP.

 

[RealiseNothing]

But then again notsureifsrs.

OP; you've only done one "part" of the question, finding k. You've made no attempt in finding the 8th term

They've given what the 8th term is.

 

[Squar3root]

They've given what the 8th term is.

This says a lot about my reading skills

 

[D94]

Find value of k for which 81920 is the 8th term of the series 5+k+80+....

I do it this way:
K/5 = 80/K
K^2 = 400
K = 20

The answer is k = 20

But i dont get why the question says 81920 is the 8th term??

HELPPPPPP !!!




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5 + 20 + 80 + 320 + 1280 + 5120 + 20480 + 81920 + ...

The reason that 81920 is given is to indicate to you whether or not it is an AP or GP. How did you know it was a GP? If you weren't told the 8th term was 81920, then you would have no indication that it was a GP.

 

[D94]

It tells you what the 8th term is so you can figure out whether or not it is AP or GP.

Yes, this.

 

[RealiseNothing]

This says a lot about my reading skills

It really does lol.

 

[trungduong12]

I actually did not pay attention to the 8th term bit
I just use the t2-t1=t3-t2
And t2/t1 = t3/t2

Thats why i figure out the k term as well as it is a GP series


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[trungduong12]

But yeah
No worries
I got it now
This is an easy question though


Anyway !!!
Thank you everyone !!!



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[aDimitri]

It tells you what the 8th term is so you can figure out whether or not it is AP or GP.

lol this is exactly what i said at the start

 

[Squar3root]

It really does lol.

It was the first few words too :/

To be fair; I only got a band 7 in Naplan Reading - just below the state average. but band 10 in Numeracy ftw

 

[D94]

I actually did not pay attention to the 8th term bit
I just use the t2-t1=t3-t2
And t2/t1 = t3/t2

Thats why i figure out the k term as well as it is a GP series


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That doesn't indicate whether it's a GP or AP.

What if the question said the 8th term was 267.5?

By your logic, k is still 20, which it clearly is not.

 

[Squar3root]

That doesn't indicate whether it's a GP or AP.

What if the question said the 8th term was 267.5?

By your logic, k is still 20, which it clearly is not.

listen to D94

 

[trungduong12]

That doesn't indicate whether it's a GP or AP.

What if the question said the 8th term was 267.5?

By your logic, k is still 20, which it clearly is not.

Yea yea
I said i got it
Lol

I was just saying that was the way i did


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