Question

Find the sum to n terms of an ap 15, 19, 23 ..........

Answer 1

$a=15 \\ \\ d=19-15=4 \\ \\ \\ S_n=\frac{n}{2}[2a+(n-1)d] \\ \\ S_n=\frac{n}{2}[2 \times 15 +(n-1)4] \\ \\ S_n=\frac{n}{2}[30+4n-4] \\ \\ S_n=\frac{n}{2}[4n+26] \\ \\ S_n=\frac{n[2n+13]2}{2} \\ \\ S_n=n[2n+13] \\ \\ S_n=\bold{2n^2+13n}$

Hope this helps. ^_^

Thank you. :-))

#adithyasajeevan