Question

I have more awful maths hw D:
What is the sum of the finite arithmetic series?
(–15) + 0 + 15 + 30 + … + 195

Answer 1

need a calculator essentially

Answer 2

I tried using Wolfram Alpha and it says "(unable to determine general term)"

Answer 3

what is the "..." stand for?

Answer 4

hmmm what do you think is the common ratio?

Answer 5

"..."
^
ellipses =P

Answer 6

oh wait

Answer 7

every term after the last one is the last term + 15

Answer 8

yes so what do you think is the common ratio?

Answer 9

(–15) + 0 + 15 + 30 + 45 + 60 + 75 + 90 + 105 + 120 + 135 + 150 + 165 + 180 + 195 = 1350

Answer 10

is this correct?

Answer 11

can work with (1+2+3+...n) ) = (n)(n+1)/2
and a multiplier in front.

Answer 12

a=-15,d=0-(-15)=15
l=a+(n-1)d
find n,then
\[Sn=\frac{ n }{2 }\left( a+l \right)\] or
\[Sn=\frac{ n }{2 }\left\{ 2a+\left( n-1 \right)d \right\}\]
Here l=195
calculate Sn.

Answer 13

what??? i thought it was + 15?

Answer 14

sum of an arithmetic sequence => \(\Large \bf S_{\color{red}{ n}}=\cfrac{{\color{red}{ n}}}{2}(a_1+a_{\color{red}{ n}})
\)

Answer 15

Yes, 1350

Answer 16

um, i don't know how to do that exactly, but i can try

Answer 17

oh, it is right?

Answer 18

yeah, it is right
thanks mate :D
wish i could give you more than just a medal