Geometric sum formula (2 Viewers)

[MrBrightside]

both formulas = the same output, can we just use the second one?

in my textbook (MIF)

Sn = (a(r^n-1)) / r - 1 for absolute value of r is > 1

OR

Sn = (a(1 - r^n)) / 1- r for absolute value of r is < 1

Now can I just use the second one, I think the two negatives make the positive output which = the same ouput as first formula.

 

[timinator1993]

Yea, i only remember one of them, just be careful about the signs thats all

 

[maxc16]

both formulas = the same output, can we just use the second one?

in my textbook (MIF)

Sn = (a(r^n-1)) / r - 1 for absolute value of r is > 1

OR

Sn = (a(1 - r^n)) / 1- r for absolute value of r is < 1

Now can I just use the second one, I think the two negatives make the positive output which = the same ouput as first formula.

Assuming you meant Sn = (a(1 - r^n-1)) / 1- r for the second one, then yes either works. Depends if r is greater or less than one, it makes it easier to choose the relevant one.

 

[MrBrightside]

Assuming you meant Sn = (a(1 - r^n-1)) / 1- r for the second one, then yes either works. Depends if r is greater or less than one, it makes it easier to choose the relevant one.

nononono I didn't mean that. Anyways I've tested it on various questions and I get the same results with either formula, esp when r is > 1, i can still use the r < 1 formula and get the right answer.

My question is would a marker bitch about it in the HSC and say you used the R < 1 formula where u should of used the r > 1 formula?

*note the text book says they are the same formula, just rearranged.

I mean it just makes it a tad easier if there's just one formula to remember becasue it relates closely to the limiting sum formula Sn = a / (1 - r)

 

[hscishard]

nice work dude, they're exactly the same thing.

Try multiplying one of the formulas with -1/-1. What do you get OMG?

 

[RawrJohnT]

I REALLY don't see how it's difficult to decide whether to have r-1 or 1-r. If you're that lazy I guess you can just select one of the formulas, but seriously..?

 

[OmmU]

I only use

BUT just remember that there is the alternate form (which isn't hard to remember). If you get a -ve answer use your common sense about the question.

 

[MrBrightside]

I REALLY don't see how it's difficult to decide whether to have r-1 or 1-r. If you're that lazy I guess you can just select one of the formulas, but seriously..?

I'm not lazy. I'm just curious as to what is right and wrong, seeming that both formulas lead to a correct solution.

 

[hscishard]

You haven't specified whether or not your question is answered.

Take your first equation
a(r^n - 1)/ [r-1]

= -a(r^n -1)/ -[r-1]

= a(1-r^n)/[1-r]

OMG, what? They're the same thing! No wonder it works whenever I use the "incorrect formula"...

 

[Drongoski]

Of course the 2 formulae are the same thing - it's just that they suggest you use the 2nd version if |r| < 1 to avoid careless mistake from negative numbers.

 

[MrBrightside]

Of course the 2 formulae are the same thing - it's just that they suggest you use the 2nd version if |r| < 1 to avoid careless mistake from negative numbers.

oh, Yeah i see now, you could get a negative "a", I was doing loan repayments, that "a" will never be negative in repayment questions.

 

[asianese]

They are 100% THE SAME. The marker cannot bitch about it because they are exactly the same! It is 'more convenient' or whatever, but both are EQUALLY as correct. I like to use the one with 1-r at the bottom because it looks nicer. Besides...just write the substitution and shove it in your calculator =D

 

[DanJohnson]

are you really that lazy? you spent more time making this thread than actually memorising both formulas

 

[funnytomato]

are you really that lazy? you spent more time making this thread than actually memorising both formulas

formulae? lolz

or you could derive it if you happen to forget the formula (as long as you don't forget how to derive it), which only takes 3 lines anyways

i don't think you would get marks deducted if you use the "wrong" one

 

[DanJohnson]

formulae? lolz

or you could derive it if you happen to forget the formula (as long as you don't forget how to derive it), which only takes 3 lines anyways

i don't think you would get marks deducted if you use the "wrong" one

straight off wikipedia:

"In mathematics, a formula (plural: formulae or formulas) is an entity constructed using the symbols and formation rules of a given logical language"

 

[Drongoski]

are you really that lazy? you spent more time making this thread than actually memorising both formulas

They are exactly the same thing.

 

[MrBrightside]

They are exactly the same thing.

I know, Thank you for establishing that fact in latex, which is what I said in my second comment, if anyone cares to read.

 

[funnytomato]

straight off wikipedia:

"In mathematics, a formula (plural: formulae or formulas) is an entity constructed using the symbols and formation rules of a given logical language"

http://en.wikipedia.org/wiki/Formula#cite_note-oxford-0
and all the lecturers say formulae as well

but it doesn't reallly bother me..

 

[hscishard]

oh, Yeah i see now, you could get a negative "a", I was doing loan repayments, that "a" will never be negative in repayment questions.

confusing

If you were willing to learn you would've tolerated this font and not latex

 

[DanJohnson]

http://en.wikipedia.org/wiki/Formula#cite_note-oxford-0
and all the lecturers say formulae as well

but it doesn't reallly bother me..

it bothered you enough to quote me out initially...