Find the sum of the first 12 terms of the sequence.

1, -4, -9, -14, . . .

Question

Find the sum of the first 12 terms of the sequence. 

1, -4, -9, -14, . . .

sleepyjess · Answer

@hartnn

hartnn · Answer

do u see any pattern in that sequence?

sleepyjess · Answer

subtracting 5

hartnn · Answer

correct.
thats your "common difference" = d 

d = 2nd term - 1st term = 3rd term - 2nd term = ...
and so on

hartnn · Answer

now use the basic sum of arithmetic series formula!
$\Large S_n = (n/2) (2a+(n-1)d)$

a= 1st term 
and d = -5
and n = 12 = number of terms!

sleepyjess · Answer

$\sf S_n = (n/2)(2a+(n-1)d)$
$\sf S_n = (12/2)(2(1)+(12-1)-5)$
$\sf S_n = 6(2+11-5)$
$\sf S_n = 6*8$
$\sf S_n = 48$

hartnn · Answer

its (n-1)d

so 
$(12-1)(-5) = 11 \times (-5) = -55$

sleepyjess · Answer

oh, I didn't realize that was multiplying, not subtracting :/

hartnn · Answer

try again? :)

sleepyjess · Answer

$\sf S_n = (12/2)(2(1)+(12-1)-5)$
$\sf S_n = (6)(2+(11)-5)$
$\sf S_n = (6)(2-55)$
$\sf S_n = (6)(-53)$
$\sf S_{12} = -318$

hartnn · Answer

$\huge \checkmark $

sleepyjess · Answer

Yay!

hartnn · Answer

go through the link of the tutorial once, that i pmed you :)

sleepyjess · Answer

:) Thank you