AS Maths Help - AQA

Willing to help anybody doing the current 2015 Maths specification on C1 and C2. Also doing M1 if anybody has any questions about that too. Submit queries below

Reply 1

Ano9901whichone

Hey, I'm PM you a question that I am having trouble with. Would you be kind enough to help me?

Reply 2

KaylaB

Original post by Ano9901whichone

Hey, I'm PM you a question that I am having trouble with. Would you be kind enough to help me?

It'd be better for the question to be posted here, or even on a new thread so that it can help others as well as checking to ensure it's right :h:

Hmm. Ok then.

Ok, but I don't want this to be moved to maths where everyone starts giving answers and hints and stuff. It's staying here yeah.
Ok.
The sum of the first

k

in a geometric progression is 180. It is further given that the sum of the first

k

terms of this geometric progression is twelve less than its sum to infinity. If the sum to infinity of the geometric progression is four times as large as its second term, use algebra to determine the value of

k

.

Now please let's just give it an hour before posting (other than OP) to give the OP a chance to help me first rather than everyone else joining in ok cool we're good then.

Reply 5

elizabethkeay

Original post by Ano9901whichone

Ok, but I don't want this to be moved to maths where everyone starts giving answers and hints and stuff. It's staying here yeah.
Ok.
The sum of the first

k

in a geometric progression is 180. It is further given that the sum of the first

k

k

.

Now please let's just give it an hour before posting (other than OP) to give the OP a chance to help me first rather than everyone else joining in ok cool we're good then.

I've also messaged this to you but thought i would post it here too. Please excuse my notation I know its incorrect but I can't work out how to write the small letters after the U's and S's at the start of the formula so please bare with me!! I have used / to represent a division sign.

I would first of all start off by writing down what the question tells you algebraically rather than in words.

The sum of the first k in a geometric progression can be written as:
Sk=180

The sum of the first k terms of this geometric progression is twelve less than its sum to infinity can we written as:
S(infinity)=192 (add 12)

The sum to the infinity of the geometric progression is four times larger as large as its second term can be written as:
U2=48 (192/48)

Then, re write them as the constituents of each formulae:
180= a-ar^k/1-r
192=a/1-r
48=ar

If you write this out properly on a piece of paper you should now be able to recognize where you can substitute letters in for numbers.

Eg. 180=a-48^k/1-r

See if you can finish the question by following this method, if you need anymore help just let me know!