Sum of Geometric

Hey guys

Can anyone work this out for me?

Find the sum of 12 Terms.
9, -13.5, 20.25.....

I have the correct working out but incorrect answer - I'm not sure how I am getting it incorrect though as the formulae and method of calculation is correct?!

I think its wrong anyway, because by actually adding up all of the 12 terms is not the answer I obtained - so tbh I dont actually know which one is correct.

My working out...

Please assist and let me know where it is I've gone wrong (if I have) so I know how to rectify it in the future and not do the same mistake.

thx

Reply 1

Swayum

18

It's r^n right? So we have a(1 - r^n) = 9(1 - (-1.5)^12), not 9(1 - -(1.5)^12).

Reply 2

JayB23

OP

yup thats correct - i get the same answer or syntax error...

Reply 3

JayB23

OP

anyone?

at least give it a go, ignoring how I've done it and see what answer you get....

Find the sum of 12 Terms.
9, -13.5, 20.25.....

Reply 4

Swayum

18

I don't think you read my post correctly.

You seem to think (-1.5)^12 = -(1.5)^12, which isn't true and is where you're going wrong.

Reply 5

JayB23

OP

EDIT

So...

the correct way to calculate is -(1.5)^12? So basically its minus the value of 1.5^12 so written like...

9(1--129.746) / 2.5
9(-128.746) / 2.5
-1158.717 / 2.5
= -463.48

Reply 6

manthew

2

(-1.5)^12 should be a positive answer since its power is even number

(-1.5)^12 = +129.7... A POSITIVE!

you know the rackets on your calculator are installed for a reason... use it!

Reply 7

JayB23

OP

manthew

(-1.5)^12 should be a positive answer since its power is even number

(-1.5)^12 = +129.7... A POSITIVE!

you know the rackets on your calculator are installed for a reason... use it!

thats how I originally did it and the answer is incorrect.

the correct answer is following how swayum said to do it.

9(1--129.746) / 2.5
9(-128.746) / 2.5
-1158.717 / 2.5
= -463.48

It is..... 9 multiplied by value of (1.5^12 - 1) which is 9(-128.746) which is (-1158.717) then divided by 2.5 which = ~-463.48

Reply 8

shawrie777

4

Swayum

I don't think you read my post correctly.

You seem to think (-1.5)^12 = -(1.5)^12, which isn't true and is where you're going wrong.

This is right: --4^2=16 but -(-4)^2=-16
Does that make sense?

Reply 9

manthew

2

JayB23

thats how I originally did it and the answer is incorrect.

the correct answer is following how swayum said to do it.

9(1--129.746) / 2.5
9(-128.746) / 2.5
-1158.717 / 2.5
= -463.48

It is..... 9 multiplied by value of (1.5^12 - 1) which is 9(-128.746) which is (-1158.717) then divided by 2.5 which = ~-463.48

(1--129.7) IS NOT -128.7 OMG... someone kill me please :facepalm:

DUDE! get your lecturer to teach you... I admit I'm incompetent at preaching you

Reply 10

DFranklin

18

JayB23

EDIT

So...

the correct way to calculate is -(1.5)^12? So basically its minus the value of 1.5^12 so written like...The problem is, it's hard to know what you mean by its here.

Let me try to be clear: (-1.5)^12 = (1.5)^12; if you are saying that (-1.5)^12 = -(1.5)^12 you are wrong.

9(1--129.746) / 2.5

This line is wrong. You should have (1-129.746) / 2.5

9(-128.746) / 2.5

And this line does not follow from the one before. 1 - (-129.746) = 130.746, not -128.746.

Also, use brackets. Expressions like 1 - -129.746 are ugly and easily misinterpreted. (I'm tempted to say that they're inherently wrong, but that might be going a little too far).

Reply 11

JayB23

OP

manthew

(1--129.7) IS NOT -128.7 OMG... someone kill me please :facepalm:

DUDE! get your lecturer to teach you... I admit I'm incompetent at preaching you

OMG....

Add the 12 terms up manually, it comes to approx -463.

The way I did it comes up with -463 using 9(-128.746) / 2.5.

How can you say its incorrect when I am obtaining the same answer as I did when manually calculating them?

Reply 12

DFranklin

18

JayB23

OMG....

Add the 12 terms up manually, it comes to approx -463.

The way I did it comes up with -463 using 9(-128.746) / 2.5.

How can you say its incorrect when I am obtaining the same answer as I did when manually calculating them?

Because (we think) you've made two sign errors that cancel.

We can't be sure that's what you've done, because it's very hard to be sure what you mean when you write 1--129.746.

Reply 13

JayB23

OP

oh I see what you mean.

So you guys point is 1--129.746 = 1+129.746?

If thats what you mean, then thats what I originally done - but it turns out incorrect.