Question

Write the sum using summation notation, assuming the suggested pattern continues.

-9 - 3 + 3 + 9 + ... + 81

Answer 1

a=-9
d=-3-(-9)=-3+9=6
l=a+(n-1)d
81=-9+(n-1)6=-9+6n-6=6n-15
6n=81+15=96
n=96/6=16
so no.of terms=16
\[S _{n}=\sum_{n=1}^{16}\left( 6n-15 \right)\]

Answer 2

it is also correct.