Question

HELP PLEASE MEDAL AND FAN
Given the arithmetic sequence -6, 1, 8., evaluate

Answer 1

\[\sum_{4}^{9}an=\sum_{1}^{9}an-\sum_{1}^{3}an=S9-S3\]
a=-6
c.d d=1-(-6)=1+6=7
\[Sn=\frac{ n }{ 2 }\left\{ 2a+\left( n-1 \right)d \right\}\]

Answer 2

\[S3=\sum~ of~ first~ three~ terms=-6+1+8=3\]
by formula
\[S3=\frac{ 3 }{ 2 }\left\{ 2*-6+(3-1)7 \right\}=\frac{ 3 }{2 }\left\{ -12+14 \right\}=\frac{ 3 }{ 2 }*2=3\]
can you find S9 by the formula?
here n=9,a=-6,d=7