Question

Help I don' t understand how to do this question! I will give medals

Answer 1

I think it is b

Answer 2

\(S = 4 + 3 + 2 + .... + \ (-14) + \ (-15)\)

but reverse it and:
\(S = (-15) + (-14) + ... + \ 3 + \ 4\)

so
\(2S = S + S = \ ...???\)

Answer 3

-15 + 4 = -11
-14 + 3 = -11

etc etc

Answer 4

yeah i tried that and i got -130

Answer 5

4+3+2+1+0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16

Answer 6

@Saiyan what would be the 1st term in the sequence?

Answer 7

what do u mean

Answer 8

-130 !!!

you have 20 terms in the sequence, right?!?!

as you're summing from n = 1 to 20

Answer 9

continue with this and you will get there

Answer 10

well, is a sequence , notice \(\large \bf \sum\limits_{n=1}^{20}\quad (5-n)\qquad thus\qquad a_1=?\)

Answer 11

\(\large a_1=first\ term=?\)

Answer 12

\(\large \begin{array}{llll}
\sum\limits_{{\color{brown}{ n=1}}}^{20}\\
\ \uparrow \\
hint
\end{array}\)

Answer 13

so that's 20 x (-11) / 2

yes?

Answer 14

yeah

Answer 15

ahemmm using \(\large S_n=\cfrac{n}{2}(a_1+a_n)\) yes