C2 Proof

How to prove the sum of a geometric sequence

i got this so far
$S_n = a+ar+ar^2 .........ar^{n-1}$

$rS_n=ar+ar^2 + ar^3 ......ar^{n-1} +ar^n$

what to do next?

Subtract the two so that most of the terms cancel out.

ok
lets call rSn 2 and Sn 1
1-2
$S_n -rS_n= a+ar^n$???

No. Try again, what does subtraction mean? Do you understand the difference between subtraction and addition? How did you get $+ar^n$?

To me

right i see so it must be $-ar^n$ thanks for clearing that up

Yep.

phew that was close almost asked for help on the integration question(8) then realised i made a mistake on the first bit
where i said

the gradient of the normal is $\dfrac{1}{2}$


when i said earlier on$\dfrac{\mathrm d y}{\mathrm d x} =-1$

at least i didn't get it wrong and the last question was a show ^-^ what a lifesaver "show" is....

Good going.

Good job.

https://b940eb267c31463d4d23ccfd3105c3b78e5616c6.googledrive.com/host/0B1ZiqBksUHNYUnQzSHRkZ3JoNGs/for-Edexcel/Solomon%20D%20MS%20-%20C2%20Edexcel.pdf
question 3

where do they get the y value of 3 from????

Uhm... you have the equation y = x + 4/x^2 that gives you the y coordinate for every x coordinate you plug in...