Question

Easy question:

Sn = 1[2(3)+(4-1)

Answer 1

\[\Large \text{\S}_n=\frac{1[2\cdot 3+(4-1)\cdot 5]}{2} \] ?? you mean like this ?

Answer 2

\[\Large \text{\S}_n \frac{6+15}{2}=\frac{21}{2}=10.5\]

Answer 3

is this sum of arithmetic sequences ??